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Textbook of General Biology. 

Cloth, 6 by 9, viii and 361 pages, 
194 figures. 1931. 






Professor of Zoology, University of Illinois 






COPYTIIOTIT, 1927, 193O, 19.35, 

All Rights Reserved 

This book or any part thereof must not 
be reproduced in any form ivithoiit 
the written permission of the publisher. 

Printed in U. S. A. 

Printing Composition and Plates Binding 




E. S. S. AND F, S. S. 


In preparing this new edition, I have taken the opportunity 
to recast and rewrite the first half of the text, incorporating such 
new material, especially in experimental vertebrate embryology, 
as I have found helpful in my own teaching, correcting errors 
which had persisted through earlier editions, and particularly re- 
vising the terminology to bring it into line with current American 

In recent years I have found that a brief preview of the life 
histories of the vertebrate types makes an admirable introduction 
to the study of comparative embryology, familiarizing the stu- 
dents with the essential vocabulary and supplying them with a 
framework for the more detailed study of development which 
follows. Such a preview is included as Chapter II. The illus- 
trations of this chapter are not labeled in detail as many of them 
will be found again with full labeling in later chapters. 

Since many of the newer textbooks of general zoology give 
excellent brief accounts of cytology and genetics, the section of 
this book devoted to these subjects now forms a separate chapter 
(IV) which may be omitted at the teacher's discretion. It has 
been completely rewritten, in which task I have been greatly 
assisted by the publication of Sharp's " Cytology," third edition. 

It is impossible, in a text designed for the college rather than 
the medical school, to ignore the new advances in experimental 
embryology, especially those directly concerned with vertebrate 
development. From the wealth of material now available in 
works of reference (see page 177) I have selected such topics as 
seemed to have a definite pedagogical value in my own experi- 
ence. It is hardly to be expected that this selection will meet 
the needs of all teachers, but it is hoped that it will supply at 
least a point of departure. With this in mind the material has 
been segregated into a single chapter (VII) and the reference list 
made a little more extensive than those in other sections of the 
book. "Embryology and Genetics" by Morgan, and "The Ele- 
ments of Experimental Embryology" by Huxley and de Beer 


have been of material assistance in the organization of this 

Earlier editions employed, to a degree which I now recognize 
was extreme for undergraduates, embryological terms current in 
European texts and manuals for advanced students. This made 
it difficult for the student to carry on collateral reading in other 
texts. In this revision the number of technical terms employed 
has been reduced until there are only 220 which are not encoun- 
tered also in such recent texts as Curtis and Outline's " General 
Zoology," second edition, and Adams 7 " Introduction to the 
Vertebrates," representing material provided in courses pre- 
requisite to embryology. A glossary, including cross references 
to common synonyms, has been added. Freed from the neces- 
sity of acquiring a large new vocabulary, the student, it is hoped, 
will progress more rapidly to a better understanding of the 
dynamic aspects of development. 

The expansion of the text indicated above has made it im- 
possible to extend the treatment of organogeny to an equal 
degree, and I have been forced to content myself with a general 
revision of this section and the substitution of new figures 
wherever better material has been available than heretofore. 

In preparing this revision, I have had the able assistance of 
Dr. Frank B. Adamstone, associated with me in this laboratory 
since 1928, and Dr. David H. Thompson, who has been good 
enough to read the new chapter on chromosomes and the genes. 
Mr. W. F. Hoheisel, my laboratory assistant, classified a great 
mass of student queries accumulated through the use of a ques- 
tion-box ever since this text first appeared in mimeograph form. 
His analysis was most revealing as to the topics where students 
encountered the greatest difficulties, and these have had special 
attention in the revision. The new drawings, except certain 
cuts borrowed from other sources and acknowledged in their 
respective legends, have been prepared by Mrs. Katharine Hill 
Paul. It is a pleasure to express my thanks to all those fellow 
teachers who have made me their debtor by suggestions as 
to how the text might be made more useful in classroom and 


January 5, 1935. 


This book is intended to serve as an introduction to the study 
of Vertebrate Embryology for undergraduate students in col- 
leges and universities. For this study they have been prepared 
by completing introductory courses in the principles of biology 
and the anatomy of the vertebrates. It has been the writer's 
aim to correlate embryological principles as discussed in lecture 
and classroom with the anatomy of vertebrate embryos as studied 
in the laboratory, in such a manner as to produce a text which 
should be both practical and teachable. 

After a general introduction to the subject, a large part of the 
text is devoted to the subject of early embryology, making use 
of the comparative method which has been found most successful 
in the experience of many teachers. Especial emphasis has been 
laid on four forms: Amphioxus, because of the beautiful and 
diagrammatic simplicity with which the early stages may be 
seriated; the frog, long an object of laboratory study; the chick, 
always available for laboratory preparation and observation; 
and man, whenever human material is available. Following this 
section, which includes the period of germ-layer formation, the 
embryonic membranes, and the development of body form, 
a second division of the book deals with the derivation of the 
separate organs and organ systems from the germ layers. Here, 
too, the method is essentially comparative. The general plan 
by which each organ system develops is first sketched in broad 
outlines, followed by an account of the divergent details in the 
frog, chick, and man. 

The remainder of the book is intended for laboratory use. 
In the third section is given a succinct account of the anatomy of 
each of the more commonly studied stages in the development of 
the frog, chick, and pig, illustrated by figures of whole mounts 
and sections selected from the splendid collections at the Uni- 
versity of Illinois built up by Professor J. S. Kingsley. The 
writer has followed the sequential or chronological method in this 
section, as it has been his experience that this method is as sue- 



cessful in the laboratory as is the comparative method in the class- 
room. It is hoped that from the study of the transparent whole 
mounts, as well as the transverse, sagittal, and frontal sections, 
the student may be enabled to visualize the anatomy of em- 
bryos in three dimensions. The concluding section of the text 
deals with methods of preparing embryos for study and of in- 
structions in elementary methods of embryological study. In 
the writer's experience it is much easier for the student to grasp 
the relationship of serial sections after he has prepared a set of 
his own. 

In view of the fact that this book has been written primarily 
for the undergraduate, the writer feels that he need offer no 
apology for the omission of historical reviews, controversial dis- 
cussions of obscure phenomena, references to original sources, 
or lists of synonyms. These neither interest nor profit the be- 
ginning student. In the concluding division of the introduction 
may be found a carefully selected list of handbooks, texts, and 
atlases to which the more ambitious student may be referred, 
while a list of references for collateral reading follows each chap- 
ter. The brevity of the text is intentional. If the student is 
informed that every word and sentence is an integral part of the 
story, he will master it in detail rather than attempt to pick out 
the more salient points, a procedure for which he is hardly pre- 
pared as yet. Brief summaries are appended to the chapters in 
Parts I and II. 

To compensate for the brevity of the text, the reader is provided 
with a profusion of illustrations, prepared by the well-known 
scientific artist, Mrs. Katharine Hill Paul. To her skill the 
writer is deeply indebted. Attention may be called to the fact 
that no abbreviations are employed in the labeling of figures. 
The beginning student will be apt to study the illustrations more 
carefully if he is not compelled to search through lengthy legends 
to interpret them. Professor E. B. Wilson and Professor J. S. 
Kingsley have kindly allowed the writer to have several figures 
from their writings redrawn in order that these might conform 
to the general style of this text. Messrs. W. B. Saunders have 
generously consented to the reproduction of certain figures, from 
" Developmental Anatomy " by Professor L. B. Arey, which 
had been drawn by Mrs. Paul for that text. The writer is 


indebted also to Professor S. H. Gage and the Comstock Pub- 
lishing Company for the use of an electrotype from " The Micro- 
scope." The source of all illustrations not original in this text 
is acknowledged in the legends. 

It is a pleasure to record here a debt of gratitude to Professor 
J. H. McGregor and Professor J. S. Kingsley for their kindness 
in reading the original manuscript. Professor H. B. Ward has 
generously placed the resources of the Department of Zoology 
at the writer's disposal during the preparation of this book. Dr. 
A. R. Cahn has contributed preparations, suggestions, and 
most appreciated of all uncounted hours in the drudgery of 
proof-reading and indexing. For assistance in these labors the 
writer is indebted also to Dr. H. W. Hann and Dr. F. B. Adam- 
stone. If this book serves to help the undergraduate through his 
first course in embryology, it is due, in no small measure, to the 
many students who have labored through these pages in mime- 
ographed form and pointed out the difficulties they encountered. 


University of Illinois, 




I. The study of embryology 3 

II. Vertebrate life histories 12 

A. Amphioxus 12 

B. Frog 16 

C. Chick 19 

D. Man 23 


III. The germ cells and fertilization 31 

A. The gametes 31 

B. Gamctogenesis 43 

C. Fertilization 50 

IV. The chromosomes and the genes 60 

A. The chromosomes 60 

B. The genes 72 

V. Cleavage and the germ layers 91 

A. Cleavage 91 

B. Gastrulation 106 

C. The middle germ layer 114 

VI. Embryonic form and extra-embryonic structures 126 

A. The form of the body 126 

B. The yolk sac 135 

C. Amnion and chorion 137 

D. The allantois 142 

E. The placenta 144 

VII. Experimental vertebrate embryology 153 

A. The organization of the fertilized egg 153 

B. Organization of the embryo during cleavage 158 

C. Organization of the embryo during germ layer formation 165 

D. Environmental factors in development 171 





VIII. Endodermal derivatives 181 

IX. Mesodermal derivatives 195 

A. The coclom and its mesenteries 195 

B. The nephric organs 199 

C. The genital organs 205 

D. The adrenal organs 213 

E. The vascular system 214 

F. The skeleton 229 

G. The muscles 238 

X. Ectodermal derivatives 244 

A. The integument 244 

B. The nervous system 247 

C. The sense organs 261 


XI. The anatomy of frog embryos 275 

A. The early embryo (3 mm.) 275 

B. The larva at hatching (0 mm.) 278 

C. The young tadpole (11 mm.) 287 

XII. The anatomy of chick embryos 294 

A. The twenty-four hour stage 295 

B. The thirty-three hour stage 298 

C. The forty-eight hour stage 303 

D. The seventy-two hour stage 309 

XIII. The anatomy of the 10 mm. pig embryo 316 


XIV. Preparation of embryological material 331 

A. Collection and rearing of embryos 331 

B. Preservation of material 333 

C. Whole mounts 336 

D. Serial sections 339 

E. Technical records 348 

F. Outline of technical procedure for chick embryos 348 

XXV. Study of embryological preparations 350 

A. The use of the microscope 350 

B. Embryological drawings 354 

C. Reconstruction 357 


INDEX 375 




Embryology may be defined as that division of biological science 
which deals with the development of the individual organism. 
It is concerned with the orderly series of changes in form and 
function through which the initial germ of the new individual is 
transformed into a sexually mature adult. Among vertebrate 
animals, at least, the germ with which development commences is 
normally an egg that has been fertilized by a sperm. The 
sexually mature adult is an individual which has developed to a 
point where it can produce mature eggs if a female, or sperms if a 
male. Sometimes the word ontogeny is used as a synonym for 
embryology, but more often it is defined to include the entire life 
history of an individual from its origin to its death. 

Early embryologists. The earliest treatise on embryology 
which has been preserved is that of Aristotle (384-322 B.C.) 
entitled " De Generatione Animalium," concerning the genera- 
tion of animals. This work describes the reproduction and de- 
velopment of many kinds of animals. It contains the first 
account of the development of the hen's egg, day by day, so far 
as it could be seen with the naked eye. Comparing the different 
types of reproduction, Aristotle placed the mammals first, for, 
being unable to discover the egg, he thought that their young 
arose from a mixture of male and female fluids and were " born 
alive.' 7 Sharks, on the other hand, arose from eggs which were 
retained within the body of the mother and so were also born 
alive. Next he placed the type of reproduction shown by reptiles 
and birds in which the egg is " complete/' that is to say, furnished 
with albumen and a shell. Lowest among the vertebrates were 



the amphibians and bony fish with " incomplete " eggs. His 
account of development showed great powers of observation, 
skill in comparison, and imagination in interpretation. Notable 
among his speculations is one which has been given the name of 
" epigenesis." From his observations on the development of the 
hen's egg he concluded that development always proceeds from a 
simple formless beginning to the complex organization of the 

Another famous name in embryology is that of William Harvey 
(1578-1657). His book (" Exercitationes de Generatione Ani- 
malium ") is largely based on the development of the chick, which 
he described in great detail, although he too was limited by the 
fact that the microscope had not yet come into general use. 
One of his contributions was a careful study of the development 
of the deer, which he compared with that of the chick. From 
purely theoretical considerations he came to the conclusion that 
mammals also formed eggs, and is responsible for the dictum 
" Ex ovo omnia " all animals arise from eggs. 

After the invention of the microscope, Marcello Malpighi 
(1628-1694) published an account of the development of the hen's 
egg (" De Ovo Incubato ")> illustrated with excellent figures of 
development from the 24-hour stage of incubation on. His work 
was responsible for a theory of " preformation " as opposed to 
Aristotle's " epigenesis." On theoretical grounds, he held that 
the various parts of the embryo were contained in the egg and 
became visible as they increased in size. The enthusiasm result- 
ing from the remarkable discoveries made with the newly invented 
microscope led to many later and wholly imaginative accounts of 
homunculi miniature adults in eggs or sperms, respectively. 
Caspar Friedrich Wolff (1733-1794), in a highly theoretical 
treatise (" Theoria Generationis "), attacked the theory of pre- 
formation on logical grounds. A more important contribution on 
the development of the intestine in the chick demonstrated that 
the tubular intestine arose from the folding of a flat laj^er in an 
earlier stage of incubation. This was a direct refutation of the 
preformationist idea that the intestine was tubular from the 

Comparative embryology. Karl Ernst von Baer (1792-1876) 
is known as the " father of modern embryology." He discovered 


the egg of mammals in 1827 and published a book on animal 
development (1828 and 1837) in which he compared in detail the 
development of different animals. From these he drew four 
important conclusions, known as von Baer's laws. 

" 1. The more general characteristics of any large group of 
animals appear in the embryo earlier than the more special 

" 2. After the more general characteristics those that are less 
general arise and so on until the most special characteristics 

" 3. The embryo of any particular kind of animal grows more 
unlike the forms of other species instead of passing through them. 

" 4. The embryo of a higher species may resemble the embryo 
of a lower species but never resembles the adult form of that 
species/ 7 

From the time of von Baer up to the present the history of 
embryology has been marked by increasing specialization. Thus 
there is a comparative embryology of the vertebrates and a com- 
parative embryology of the invertebrates. There are also other 
divisions of embryology which will be indicated briefly in the 
following paragraphs. 

Cellular embryology. Soon after the first volume of von 
Baer's treatise appeared, Schleiden and Schwann (1838, 1839) 
announced the cell theory, namely that all living things are 
composed of, and arise from, living units known as cells. This 
resulted in an intensive study, commencing in the latter part of 
the nineteenth century, of the germ cells, their origin and fertil- 
ization, which led Sutton in 1901 to the chromosomal theory of 
inheritance. In 1878 Charles Otis Whitman (1842-1910) traced 
for the first time the detailed history of the cells formed by the 
dividing egg (in the leech, Clepsine), thereby initiating the study 
of cell lineage. Cellular embryology is a subject which unites 
embryology with cytology, the study of the cell and its activities. 

Genetics and embryology. In 1866, Gregor Mendel (1822- 
1884) first carried on successfully experiments in breeding plants 
to discover the laws by which individual characteristics are in- 
herited from one generation to another. His conclusions, now 
known as Mendel's laws, will be discussed in a later chapter, 
contributions were long unrecognized, but in 1900 they were 


rediscovered and in the following year Sutton first suggested that 
the behavior of the chromosomes afforded a mechanical explana- 
tion of these laws. This has led to the theory of the gene, a name 
proposed for the unit of heredity by Johannsen (1911). This 
theory in the hands of T. H. Morgan has assumed great impor- 
tance to the embryologist, for, to quote from Brachet, "Embry- 
ology is fundamentally the study of heredity in action." 

Phylogeny and embryology. In 1866, Ernst Haeckel (1834- 
1919) published a theory which he believed supported Darwin's 
theory of evolution and which he called the " fundamental bio- 
genetic law." It is more often known as the recapitulation 
theory. This theory states that ontogeny is a brief and incom- 
plete recapitulation of phylogeny, or that an animal passes 
through stages in its development comparable to those through 
which its ancestors passed in their evolution. So far as the 
vertebrates are concerned, this would mean that a mammalian 
embryo should pass through stages which are definitely fish-like 
and later through stages which are essentially reptilian. The 
fact is that, although there are individual characteristics which 
at times are reminiscent of fish-like or reptilian ancestors, there 
is never a time in the development of a mammal when it could 
be mistaken for a fish or a reptile. There are evidences that the 
vertebrates do retain in development certain features which also 
appeared in the development of their ancestors. For example, 
clefts appear in the pharynx of the embryos of birds and mammals, 
opening to the exterior just as they do in the embryos of fish. In 
the adult fish these clefts contain the gills, but this is not true of 
adult reptiles or birds. It has been found very difficult, if not 
impossible, to draw up a genealogical tree of the vertebrates 
based solely on embryological data, and the recapitulation theory 
is not so widely accepted as in former times. 1 

Experimental embryology. Among Haeckel's contemporary 
opponents was Wilhelm His (1831-1904), who directed attention 
to the physiology of the embryo. Denying the theory of reca- 
pitulation, he called attention to the mechanical processes by 
which the various structures of the embryo arise from particular 
regions of the germ. Later, Wilhelm Roux (1850-1924) put the 
study of experimental embryology on a firm basis when he pub- 
1 Shumway. 1932. " The Recapitulation Theory/' Quart. Rev. Biol. 7:93-99. 


lished a program for the new science which he called " the 
mechanics of development/' This has led to an intensive attack 
upon the problems of development from the physico-chemical 
side which is carried on actively at the present time. Weismann 
(1834-1913), a leader in theoretical embryology, suggested a 
theory of chromosomal inheritance which came very close to the 
mark. Jacques Loeb (1859-1924) discovered a method of induc- 
ing development in unfertilized eggs (artificial parthenogenesis) 
which has led to extensive research on the nature of fertilization. 


Embryology in the classic period 
4th century B.C. Aristotle 

Embryology in the Renaissance period 
(Before the general use of the microscope) 
1651 Harvey Epigenesis 

(After the general use of the microscope) 

1672 Malpighi Preformation 

1768 Wolff Epigenesis 

Embryology in modern times 

1839 von Baer Comparative embryology 

1839 (Schleiden and Schwann announced cell theory) 

1859 (Darwin announced theory of natural selection) 
1866 Haeckel Biogenetic law 

1866 (Mendel announced laws of inheritance) 

(Microscopic technique being developed) 

1874 His Experimental embryology 

1878 Whitman Cell lineage 

1883 Roux Mechanics of development 

1891 Weismann Theory of the germplasm 

1899 Loeb Artificial parthenogenesis 

1900 (Rediscovery of Mendel's laws) 

Chemical embryology. No attempt is made to mention the 
names of men still alive who have contributed to our knowledge 
of embryology, for the roll of distinguished zoologists here and 
abroad would have to be called. Yet it may be admissible to 
comment on the recent appearance of a monumental work in 
three volumes by Joseph Needham, entitled "Chemical Embry- 


ology," which seems to chart the course for still a new subscience 
in embryology. 

The value of embryology. To the student who specializes in 
zoology, embryology has a particular importance because it deals 
with the origin and development of the adult body. There is a 
fascination in tracing out the history of the different anatomical 
structures as they take form, grow, and gradually assume the 
appearance familiar to us in the mature animal. And in the 
history of the different organs are found clues to their relation- 
ships and functions. Everyone knows, for example, that the 
adrenal gland secretes a hormone, epinephrin, which, circulating 
in the blood, rouses the autonomic nervous system to greater 
activity. But in embryology the student learns that the part of 
the adrenal gland which secretes epinephrin is derived from those 
same ganglia which give rise to the autonomic nerves. 

He also finds clues to ancestral relationships. Even though the 
recapitulation theory has been abandoned as an explanation of 
development, embryologists recognize that there are structures 
in the body which correspond to similar parts used for the same 
or different purposes in the bodies of some distant ancestor. The 
" retention theory " has been proposed by de Beer as an explana- 
tion. Thus the vestigial tail of the human embryo arises in the 
same place and manner as the tails of other vertebrates, and we 
have no doubt but that some remote prehuman ancestor sported 
and made use of a tail. The embryo retains this tail, not as a 
recapitulation of ancestral history, but because it inherits the 
genes which initiate the development of a tail. So the student 
of comparative anatomy often turns to embryology hoping to find 
homologies in the mode of origin and manner of development of 
the adult organs in which he is interested. 

The modern student of embryology is concerned mainly with 
the dynamics of development. He examines the protoplasmic 
organization of the egg and the sperm, their genetic constitution, 
and the nature of the process in which the sperm initiates develop- 
ment in the egg. He traces the history of the different cells into 
which the egg divides and tries to learn the way in which that 
differentiation takes place. He is interested in the mechanics of 
the processes by which these cells arrange themselves into the 
different germ layers, and how the different organs arise. 


To these problems he brings the methods of descriptive embry- 
ology : the delicate technique of preparing ernbryological material, 
the skilled use of the microscope and its accessories, the inter- 
pretation and reconstruction of his prepared material. He also 
uses the methods of experimental embryology: the alteration of 
the normal conditions of development; new genetic complexes; 
altered environmental conditions at different stages of develop- 
ment; the development of individual cells or parts of the embryo 
in isolation or transplanted into new positions or different hosts. 

Embryology is not an easy subject. It requires a high type of 
visual imagination. The student must bear in mind that he is 
dealing with living objects, three-dimensional and continually 
changing in volume, shape, and constitution. Much of his at- 
tention must be given to the cells of the embryo, as they multiply, 
migrate, take on different appearances, and carry on different 
functions. But he must always remember that the embryo has 
a life of its own to lead. All the different cells and cell groups in 
the embryonic body work in harmony if the development of the 
embryo is normal. He must not lose sight of the embryo-as-a- 

The student preparing for medicine has a professional interest 
in embryology. Teachers of human anatomy have long since 
agreed that a knowledge of embryological relationships is the best 
possible preparation for the study of human anatomy. A good 
working acquaintance with the outlines of human embryology is 
prerequisite to the study of obstetrics. And the practitioner of 
medicine must be prepared to answer all sorts of questions about 
human development. 

There are two different ways of approaching the subject of 
vertebrate embryology, when more than one type of development 
is to be studied. By the first method the different types of 
development are taken up one after another, e.g., amphioxus, 
frog, chick, man. The second method consists of discussing the 
different topics of embryology in turn and comparing the condi- 
tions found in each of the types. In this book, the second, or 
comparative, method is employed. But before taking up the 
first topic in comparative embryology, it is helpful to examine, 
very briefly, the life history of each of the types to be used in 
the later discussion. This will serve to introduce the main stages 


of embryology, and to point out the different conditions under 
which development takes place. 


The history of embryology has passed through three phases. 
First came the period of fact-finding or description. The first 
name associated with this period is that of Aristotle. Before the 
invention of the microscope, Harvey, and after its invention, 
Malpighi, made careful studies of the development of the hen's egg. 

The second period is that of comparative embryology com- 
mencing with von Baer. Comparative embryology has been in- 
fluenced in the past by Haeckel's theory of recapitulation, which 
was supposed to support the Darwinian theory. With this period 
we associate also the subject of cellular embryology growing out 
of Schleiden and Schwann's cell theory. This subject is now 
closely linked with genetics, for the gene, or unit of genetics, is 
located in the nucleus of the cell. 

The present period may be called that of experimental embry- 
ology, foreshadowed by His and put on a firm basis by Roux. 

The study of embryology is of value in understanding the rela- 
tionships of the parts of the adult body, and the homologies of 
adult organs in different groups of animals. But its immediate 
aim is to discover the nature of developmental processes. Its 
methods are observational and experimental. It is concerned 
both with the behavior of the cells of the embryo and with the 
activities of the embryo as a whole. 


X/ocy, W. A. 1915. Biology and Its Makers, 3rd Ed. 
v Needham, J. 1931. Chemical Embryology, Vol. 1. 

1934. History of Embryology. 

Nordenskiold, E. 1928. The History of Biology. 

Russell, E. S. 1916. Form and Function. 

Collateral textbooks. 

$ee also references, Chap. II. 

-.,'McEwen, R. 8. 1931. Vertebrate Embryology, 2nd Ed. 
Richards, A. 1931. Outline of Comparative Embryology. 

Wieman, H. L. 1930. An Introduction to Vertebrate Embryology. 

Comparative vertebrate embryology. 
Brachet, A. 1921. Traite* d'embryologie des vertebres. 


Hertwig, O. (ed.) 1900). Handbuch dcr vergleichenden und experimentallen 

Entwickelungslehre dcr Wirbelticre. 6 volumes. 
Jenkinson, J. W. 1913. Vertebrate Embryology. 
Kellicott, W. E. 1913. Chordate Development. 
Kerr, J. G. 1919. Textbook of Ernbrvology, Vol. 2., Vertebrates exclusive of 


Comparative invertebrate embryology. 

Dawydoff, C. 1928. Traite d'crnbryologie comparee des invertcbres. 
Korschelt, E., and Heider, K. 1902-1910. Lehrbuch der vergleichenden Ent- 

wicklungsgcschichte dcr wirbellosen Tiere. 
MacBride, E. W. 1914, Textbook of Embryology, Vol. 1., Invertebrates. 

Cellular embryology (and cytology). 

Cowdry, E. V. (ed.) 1924. General Cytology. 

Sharp, L. W. 1934. Introduction to Cytology, 3rd Ed. 

Wilson, E. B. 1925. The Cell in Development and Heredity, 3rd Ed. 

Experimental embryology. 

See references. Chap. VII. 


Duval, M. 1884. Atlas d'crnbryologie. 

Goette, A. 1874. Atlas zur Entwickelungsgcschichte der Unke. 

Keibel, F. (ed.) 1897-1923. Normentafeln zur Entwicklungsgeschichte der 

Kohlmann, J. 1907. Handatlas der Entwickelungsgeschichte des Menschen. 


See also bibliographies in references cited above. 
Biological Abstracts, commencing with literature of 1926. 
Concilium Bibliographicum, card index, commencing with literature of 1896. 
Minot, C. S. 1893. A Bibliography of Vertebrate Embryology. Mem. Boston 
Soc. Nat. Hist., 4:487-614. 

Monographs, embryological series. 

Carnegie Institution of Washington. Contributions to Embryology. 


It is obvious that in an introductory text it is impossible to 
describe the development of an animal representing each verte- 
brate group. There are, however, four vertebrates whose embry- 
ology has been studied more intensively than any others. These 
are the amphioxus, the frog, the chick, and man. But before 
continuing with a brief account of the life history of these verte- 
brates it is advisable for the student to recall the list of terms 
which will be used in the descriptions following. 


(Synonyms in parentheses) 

Anterior (cephalic, cranial, rostral) head end. 

Posterior (caudal) tail end. 

Dorsal back side. 

Ventral belly side. 

Lateral either right (dextral) side, or left (sinistral) side. 

Mesial (median, medial) middle. 

Proximal nearer the point of reference. 

Distal further from the point of reference. 

Transverse (horizontal) a plane intersecting the antero-posterior axis at right 

angles, dividing anterior portion from posterior. 
Sagittal the mesial plane of the body or any plane parallel to it, dividing right 

portion from left. 
Frontal (coronal) any plane at right angles to both transverse and mesial planes 

dividing dorsal portion from ventral. 
Primordium (anlage, Germ.; 6bauche, Fr.) the first recognizable stage in the 

development of any new part of the embryo. 

Invagination the growth of a surface in (toward the point of reference). 
Evagination the growth of a surface out (away from the point of reference) . 


The amphioxus (Branchiostoma lanceolatum) is not really a 
vertebrate, .for it lacks a skull and vertebral column. But be- 
cause it has a notochord and other chordate structures it is a 
relative of the vertebrates, a protochordate. B^some it is be- 

12 ^ 


ture, the blastopore, which later narrows as the gastrula increases 
in length (Fig. II). 

The germ layers. The outer layer of the gastrula is known 
as the ectoderm; the inner one is called the endoderm. It really 
includes not only the endoderm proper, which is to become the 
lining of the digestive tube, but also the middle germ layer or 
chorda-mesoderm, which now occupies the roof of the gastrocoel. 

The roof of the gastrocoel soon, 11 or 12 hours after fertilization, 
develops three longitudinal grooves. The middle one of these 
folds is to become the notochord; the others give rise each to a 
series of pouches or enterocoels (Fig. 1J), from which the meso- 
derm is formed. 

The ectoderm immediately over the roof of the gastrocoel is 
called the neural plate. About 15 hours after fertilization, ecto- 
derm from the ventral side grows up over the blastopore to cover 
the neural plate (Fig. U). 

Hatching. The embryo escapes from its egg envelopes at 
this time, if not indeed a little earlier. It is now cylindrical in 
form, flattened on the dorsal surface, its length about twice its 
diameter. It appears to be about twice the volume of the 
original egg, owing to the large digestive cavity arising from the 
gastrocoel. The blastopore is covered by ectoderm from the ven- 
tral side, but this opens to the exterior by means of the an- 
terior neuropore (Fig. IK). 

The larva. After hatching, the embryo still subsists on the 
remainder of its yolk until the mouth opens, about the fourth day 
after fertilization. It is then about 1 mm. in length, very slender, 
and probably of no greater volume than the original egg (Fig. 1L). 
So soon as the mouth opens and the embryo is able to ingest food 
from external sources it is called a larva. By now all the organ 
systems except those connected with reproduction are functioning. 
For about three months the larva leads a free-swimming existence, 
making its way to deeper waters (Fig. 1M). 

Metamorphosis. At the end of three months, roughly speak- 
ing, the larva has increased in length to an average of 3.5 mm. 
It now gives up its free-swimming life to burrow in the sands and 
slowly assume its adult characteristics (Fig. IN). The ability to 
produce mature germ cells is first manifested when the animal is 
about 200 mm. long. 



The frog (Rana pipiens) is one of the anuran amphibia. It is 
selected as a type of ichthyopsid (fish and amphibia) or anamniote 
(developing without an amnion). It has been a favorite object 
of embryological observations and experiments for centuries, 
arid its development is better known than that of any other 
vertebrate except the chick. 

Spawning. The breeding season of the frog is in the early 
spripg, soon after the ice is off the ponds. The males, emerging 
first from hibernation, make their way to the breeding grounds, 
where they congregate and sing in chorus while awaiting the 
coming of the females. On arrival, each female is seized by a 
male who grasps her for long periods (amplexus). In the early 
morning both individuals discharge their germ cells, so that fer- 
tilization is external. The egg, about 1.7 mm. in diameter 
(Fig. 2 A), is surrounded with a layer of albumen which swells 
rapidly, causing the eggs, in masses of 3500 to 4500 (Wright), to 
adhere to vegetation or to rest on the bottom in shallow water. 
The yolk, present in the form of platelets, is concentrated in the 
lower hemisphere of the egg. 

Fertilization. Fertilization is external, but the close contact 
of the individuals during amplexus ensures that the sperm enters 
the egg before the swelling of the egg jelly prevents it. The first 
polar body is formed before fertilization, the second afterwards. 

Cleavage. The rate of cleavage depends upon the tempera- 
ture, but the first division (Fig. 2B) may occur from one to two 
hours after fertilization, earlier at high temperatures, later at low 
ones. Cleavage divides the egg completely, but the third cleav- 
age is unequal so that the four J-blastomeres of the animal hemi- 
sphere are markedly smaller than the four of the vegetal hemi- 
sphere (Fig. 2C). After the third cleavage, the pattern becomes 
more irregular. 

Blastula. The presence of the large yolk-laden blastomeres 
in the vegetal hemisphere results in an eccentrically placed 
blastocoel. Furthermore, cleavage planes tangential to the sur- 
face of the blastula produce a layer of blastomeres several cells in 

Gastrula. The presence of great amounts of yolk prevents 
any imagination, and gastrulation takes place by overgrowth 



K-H1U1 Jel 

FIG. 2. Development of the frog. A, fertilized egg, from side, X8. B, first 
cleavage, from posterior side, X15. C, third cleavage, from left side, X15. 
D, early gastrula, (dorsal lip stage). E, middle gastrula, (lateral lips). F, late 
gastrula, (yolk plug stage). G, neural folds. (D-G, from posterior side, semi- 
diagrammatic, approx. X15.) H, neural tube closed, 2.4 mm. I, embryo of 
3 mm. J, embryo of 6 mm. K, embryo of 9 mm. L, embryo of 1 1 mm. (H-L, 
from left side, measured alive, drawn after preservation X10.) M, full grown 
tadpole. N, after metamorphosis. (M and N, from left side, XI, after Wright, 


instead. Commencing from a shallow groove just below the 
equator (Fig. 2D), the smaller cells grow down over the larger 
ones. The down-growing cells form a two-layered fold because 
the cells at the margin are turned in as the fold grows down. 
As this overgrowth and tucking-in continues, the fold extends at 
its two extremities to become crescent-shaped (Fig. 2E). Finally 
the two ends of the crescent meet to form a circle which rapidly 
diminishes in circumference until finally only a small plug of the 
larger yolk-laden cells protrudes (Fig. 2F). The circle is known 
as the blastopore, and the groove with which it commenced is 
known as the dorsal lip of the blastopore. A gastrocoel is formed 
between the large yolk-laden cells and the smaller cells turned in 
at the lips of the blastopore. 

The germ layers. The cells which were left on the exterior 
of the gastrula make up the ectoderm, while the inturned cells 
form an inner layer which includes both the endoderm and the 
chorda-mesoderm. The roof of the gastrocoel, therefore, is made 
up of two layers. The lower layer is the endoderm; the upper 
layer, lying beneath the ectoderm, is the chorda-mesoderm. It 
separates into three longitudinal strips, of which the middle one 
becomes the notochord. The others give rise to the mesoderm, 
which grows down between ectoderm and endoderm. No en- 
terocoels are formed, but the mesoderm breaks up into a series 
of block-like somites on either side of the notochord. 

The neurula. The neural plate lies over the roof of the 
gastrocoel. It forms around its margin neural folds (Fig. 2G) 
which will later grow together to produce the neural tube. At 
this time the frog embryo is called a neurula. While the neural 
tube is being formed the embryo increases in length to about 

2 mm. This length is attained, ordinarily, on the second day 
after fertilization, although at room temperature development 
proceeds more rapidly. 

Hatching. During the first 13 to 20 days the embryo in- 
creases rapidly in length and in the development of its organ sys- 
tems and finally, when it attains the length of 6 mm. (Fig. 2J), it 
wriggles out of its jelly. At room temperature this may take 
place within 5 days of fertilization and when the embryo is only 

3 mm. in length (Fig. 21). The embryo as yet has no mouth or 
external gills but is provided with a sucker by which it attaches 


and activities quite unlike those of the adult. This larval period 
is terminated by a sudden metamorphosis associated with a 
change to terrestrial conditions. 

The egg of the chick is enormous because of the great amount 
of yolk and the albumen enclosed within the shell. Fertilization 
is internal and prior to the formation of the albumen and shell. 
Development is very rapid and accompanied by the development 
of an amnion or water bath, and an allantois which serves as an 
extra-embryonic bladder, lung, and albumen sac. These features 
are correlated with the terrestrial environment of the developing 
egg. Eggs of this type are termed " cleidoic " (Needham). 

The human egg is very small owing to the small amount of con- 
tained yolk. Fertilization is internal, and the developing egg 
soon implants itself in the wall of the uterus where its ten months 
of development proceed. The early stages of development are 
passed through very rapidly, and the blastula and gastrula are 
quite unlike any seen in other classes of vertebrates. An amnion 
is formed around the developing embryo, and this structure is 
concerned in the formation of an umbilical cord connecting the 
embryo to the placenta, a disc-shaped organ of maternal and 
fetal origin. The placenta serves as an organ of interchange 
between mother and young up to the time of birth. Develop- 
ment continues long after this event. 

The amphioxus. 

Conklin, E. G. 1932. The Embryology of Amphioxus. Jour. Morphol. 54:69- 

1933. The Development of Isolated and Partially Separated Blastomerea of 

Amphioxus. Jour. Exptl. Zool. 64:303-375. 
Kerr, J. G. 1919. Textbook of Embryology, II, Chap. 1. 
MacBride, E. W. 1915. Textbook of Embryology, I, Chap. 17. 
Willey, A. 1894. Amphioxus and the Ancestry of the Vertebrates. 

The frog. 

lluxley, J. S., and de Beer, G. R. 1934. The Elements of Experimental Embry- 
ology, Chap. 2. 

Marshall, A. M. 1893. Vertebrate Embryology. 

Morgan, T. H. 1897. The Development of the Frog's Egg. 

^[oble, J. K. 1931. The Biology of the Amphibia. 

Wright, A. H. 1914. North American Anura. 

Ziegler, H. E. 1902. Lehrbuch der vergleichenden Entwickelungsgeschichte der 
niederen Wirbeltiere. 


The chick. 

Duval, M. 1889. Atlas d'embryologie. 

Liilic, F. R. 1919. The Development of the Chick, 2nd Ed. 
XPatten, B. M. 1929. The Early Embryology of the Chick, 3rd Ed. 


Arey, L. B. 1934. Developmental Anatomy, 3rd Ed. 

Keibel, F., and Mall, F. P. 1910. Human Embryology. 

Kollrnann, J. 1907. Handatlas der Entwickelungsgeschichte dcs Menschen. 


The germ with which the development of the vertebrate com- 
mences is the fertilized egg, or zygote. Before discussing the 
development of the zygote, it is advisable to examine the gametes, 
egg and sperm, whose union results in its existence. We shall 
proceed first to the description of the gametes, comparing them 
with each other and with a generalized cell. Next we shall con- 
sider the way in which the germ cells originate and become 
mature. Thereafter we shall turn to the study of fertilization. 


Vertebrates are characterized by the bisexual method of re- 
production, in which there are two distinct sexes: the female, or 
egg-producing individuals; and the male, or sperm-producing 
individuals. Among the protochordates (tunicates) we find 
groups in which the same individual produces both eggs and 
sperms. Such individuals are called hermaphrodites. This phe- 
nomenon is rare among the vertebrates and is not typical of any 

The two kinds of gametes, eggs and sperms, differ from each 
other in appearance, size, and structure. These differences will 
be more apparent after a brief review of cell structure in general. 


A. Nucleus (composed of karyo plasm). 

1. Reticulum (composed of chromatin). 

2. Karyolymph (nuclear sap). 

3. Nucleolus (plasmasome). 

4. Nuclear membrane. 

B. Cytosome (composed of cytoplasm). 

1. Hyaloplasm (ground-protoplasm). 

2. Centrosomes (centrioles) . 

3. Mitochondria (chondriosomes). 

4. Golgi bodies (dictyosomes). 

5. Plastids. 

6. Metaplasm (relatively lifeless accumulations). 

7. Plasma membrane. 

C. Envelopes or matrix (cell wail). 




The cell. The familiar definition of a cell (Fig. 5) is, " a mass 
of protoplasm, containing a nucleus, both of which have arisen by 
the division of the corresponding elements of a preexisting cell, 3 ' 
Protoplasm in this sense refers to the living substance of the cell, 
including both the material inside the nucleus and that in the 
cell body or cytosome. It is customary to use the term karyo- 
plasm (nucleoplasm) for the nuclear protoplasm, and the word 
cytoplasm for the protoplasm of the cell body. Some writers 

Golgi bodies 



Chromatin knot 




Cell wall 

FIG. 5. Diagram of a composite cell. (After Wilson.) 

employ only the words nucleus and cytoplasm to distinguish 
between nucleus and cell body. 

The nucleus. The cell nucleus is generally a rounded body 
separated from the cytosome by a delicate nuclear membrane. 
Within this is a transparent ground substance known as the kary- 
olyrnph or nuclear sap. But the characteristic substance of the 
nucleus is its chromatin, a substance staining sharply with basic 
dyes, and arranged usually in a network of threads called the 
reticulum (Sharp). Sometimes swellings, chromomeres, are ap- 
parent at the nodes of the network. The nucleus usually con- 
tains a smaller body known as the nucleolus, a droplet of some 


material heavier than the nuclear sap, but staining with acid 

dyes. Its staining properties alter during cell division. 

The nucleus may fragment to form polyiiuclear cells. It may 
also divide, often many times, while the cell body remains un- 
divided, resulting in the formation of a syncytium. Sometimes 
the nucleus may be ejected to leave enucleate cells such as the red 
blood corpuscles of mammals. But in general every cell has one 

The type of nucleus here described is known as the vesicular 
type. There is distinguished also the massive or compact type of 
nucleus, in which the chromatin forms apparently a solid mass, 
as in the sperm cell. Then there is a diffuse type, in which the 
nuclear membrane is absent and the chromatin is scattered 
through the cell body in granules called chromidia. 

The cytosome. The cytoplasm of the cell body includes an 
outer delicate semipermeable membrane known as the plasma 
membrane. This is the surface at which the protoplasm of the 
cell is in contact with its environment. Within this is the liquid 
ground substance or hyaloplasm, in which are distributed a 
number of differentiated bodies. Of these cytoplasmic inclusions 
the more important seem to be the centrosomes, mitochondria, 
and Golgi bodies, all of which appear to have the properties of 
independent growth and division. 

The centrosomes (centrioles), small spherical bodies, one or two 
in number, lie near the nucleus. They seem to be concerned in 
the process of cell division. In cells with locomotor organs, like 
the tail of the sperm, the centrosomes are connected with the 
contractile element of the cell. 

The mitochondria (chondriosomes) are small rods, or granules, 
very numerous and scattered through the cytoplasm. They are 
dissolved by many common methods of preparing cells for obser- 
vation, but can be demonstrated in the living cell by a stain 
called Janus Green B. They are preserved by special chemicals, 
e.g., osmium tetroxide. 

The Golgi bodies (dictyosomes) are sometimes scattered through 
the cytoplasm but often aggregated into a network, the Golgi ap- 
paratus. Some authors deny that there is a real structure of 
Golgi bodies and speak therefore of the Golgi material or the 
Golgi zone. Other investigators have sought to identify these 


bodies with the plastids, cytoplasmic elements which are found in 
plant cells. Golgi bodies are hard to identify in living cells but 
can be demonstrated by special techniques involving the use of 
osmium tetroxide or silver nitrate. Their function is doubtful, 
but there is some reason to believe that they are concerned with 
the elaboration of substances within the cell such as enzymes. 

Still another type of inclusion in the cytosome is represented 
by the plastids. These bodies are found more frequently in plant 
cells, e.g., chloroplasts, the chlorophyll bodies, which appear to 
have the capacity of independent growth and division. 

Metaplasm is the name given to all those bodies in the cyto- 
plasm which clearly do not possess the properties of independent 
growth and division. These may be aggregated in vacuoles or 
distributed in tiny droplets, granules, etc. Among these are 
such bodies as secretory granules, intermediate stages in the pro- 
duction of cell secretions (enzymes, etc.). Storage granules are 
end stages in the accumulation of reserve food materials such as 
yolk, oil, starch, etc. Here also we may include the minute 
pigment granules. Embryologists sometimes use the term deuto- 
plasm for reserve food materials in the cell. 

The cell wall. In concluding this brief review of cell struc- 
tures we must recall that the cell may secrete a wall around itself 
such as the vitelline membrane. In some tissues these cell walls 
unite to form a matrix such as the intercellular substance of 
cartilage or bone. 

The sperm. The male germ cell of vertebrates is a very 
minute flagellate cell ranging in size from 20 microns (crocodile) 
to more than 2 mm. (Discoglossus, an amphibian). The general 
shape is that of a tadpole with an excessively long tail, but there 
are sufficient differences among these tiny cells for them to be 
identified by specialists. 

The sperm (spermatozoon) consists of a head and a tail (Fig. 6). 
The head contains the nucleus, which is compact and stains very 
deeply with basic dyes. Here also is the acrosome, usually at the 
apical end, originating from Golgi bodies, possibly connected with 
the production of some secretion involved in fertilization. The 
head is surrounded with a delicate plasma membrane. 

The tail consists of three divisions: middle-piece, main-piece, 
and end-piece. The middle-piece contains two centrosomes. 


Yolk. The bulk of the egg is due to the presence of meta- 
plasm in the form of yolk. This substance contains the principal 
foodstuffs for the developing embryo. Studies on the yolk of the 
hen's egg indicate that it contains water (50 per cent), proteins, 
fats, carbohydrates, inorganic salts, vitamins, pigments, and 
enzymes (Needham). 

The yolk is present in the form of spheres, ovoids, or discs, which 
stain usually with basic dyes. The yolk tends to accumulate in 
one hemisphere of the egg, forcing the nucleus into the other. 
Since the yolk is heavier than the other constituents of the egg, the 
yolk-laden hemisphere is the lower one when the egg is suspended 
in water. In large-yolked (macrolecithal, megalecithal) eggs, 
such as those of the frog and chick, the accumulation of the yolk 
in one region is so marked that they are known as telolecithal eggs. 
In small-yolked (microlecithal, oligolecithal) eggs, like those of the 
amphioxus and of man, the yolk is distributed more generally and 
they are called isolecithal (homolecithal). 

Polarity. Even in isolecithal eggs there is a visible distinction 
between the two hemispheres of the egg, so that an axis exists 
from the center of one hemisphere to that of the other. This, 
known as the polar axis, is the earliest indication of a differentia- 
tion in the egg. The two ends of the axis are known as the poles. 
The polar bodies, referred to in the preceding chapter, are formed 
at one of these which is known as the animal (apical) pole. It is 
sometimes called simply the pole. The other is called the vegetal 
(vegetative, abapical) pole, sometimes the antipole. The nucleus 
always lies in the polar axis, more or less towards the animal 
pole. The yolk shows a gradation from the animal towards the 
vegetal pole. We shall observe in later chapters that the animal 
pole marks the anterior end of the developing embryo and the 
vegetal pole marks the posterior end. There is also reason to 
believe that the polar axis, in addition to being the first expression 
of symmetry in the egg, marks a gradient of metabolism (Child). 
By this is meant that metabolic processes are accelerated at the 
animal pole and progressively retarded towards the vegetal pole. 

A considerable body of evicltence shows that the animal pole of 
the egg is the one which was most active in physiological exchange 
with its environment while still in the ovary. It is the pole of the 
egg which is attached to the ovary in the amphioxus (Conklin) 



and the chick (Conklin). It has been suggested that in the frog 
the animal pole of the egg is the one lying nearest the arterial 
blood supply (Bellamy). 

Egg envelopes. The ovum usually possesses, in addition to 
the plasma membrane, a variety of protective envelopes which 
are divided into three classes according to the mode of their for- 
mation. Primary envelopes are those formed by the egg itself, 
such as the delicate vitelline membrane. The secondary enve- 
lopes are those formed by the follicle cells which immediately sur- 




FIG. 9. Egg of Myxinc, showing "chorion" and micropyle (after Dean). 

round the egg in the ovary. A good example is the so-called 
" chorion " of one of the cyclostomes, Myxine (Fig. 9). It is 
usually quite difficult to distinguish primary from secondary en- 
velopes, and it is probable that many vitelline membranes are 
compound in origin. In those vertebrates in which fertilization 
is external, such as the cyclostomes and bony fish, the primary 
and secondary envelopes are often perforated by openings called 
micropyles through which the sperm may have access to the egg. 
The tertiary envelopes include all those formed by the walls of the 
oviduct during the passage of the egg. Examples are the egg 
albumen, shell membranes, and shells of such groups as the rep- 
tiles, birds, and the egg-laying mammals; the egg capsules of the 
elasmobranchs, and the egg jelly of the amphibia and many bony 



fish. These envelopes are not formed until after fertilization, 
except in the case of the egg jelly, and this does not attain its 
final thickness until after the entrance of the sperm, when it 
swells by the absorption of water. 

THE EGG OF THE AMPHioxus. The eggs (Fig. 10A) are 0.1 
mm. in diameter. Before maturation the large nucleus is roughly 
0.05 mm. in diameter displaced well towards the animal pole. 
The cytoplasm consists of a thin outer layer relatively free from 




2nd Polocyte 



2nd Polocyte 
Egg jelly 
Egg jelly 


Corona radiata 
(follicle cells) 

1st Polocyte 


Zbna pellucida 

FIG. 10. Typical eggs. A, amphioxus, approx. X70 (after Wilson in Willey). 
B, frog X8. C, hen Xj (after Duval). D, human X250 (after Allen in Arey). 

yolk, and probably containing mitochondria. The rest of the 
cytoplasm contains yolk. There are no egg envelopes except 
perhaps a vitelline membrane. The egg is classed as isolecithal. 
THE EGG OF THE FROG. The diameter of the egg (Fig. 10B) 
is 1.7 mm. (R. pipiens, Wright), with a large nucleus before 
maturation. There is a thin outer layer of cytoplasm, containing 
granules of pigment in the animal hemisphere. Pigment is also 
found around the nucleus. The yolk is distributed with fewer 
and smaller platelets in the animal hemisphere grading down to 
more and larger platelets in the vegetal hemisphere. There are 
a vitelline membrane (primary), " chorion " (secondary), and one 
to three layers of egg jelly (tertiary). The eggs are discharged 




Outer shell 


Inner shell 



FIG. 11. Diagram of hen's egg sectioned 
(after Lillie). 

in large masses which adhere to each other by means of this jelly. 
The eggs are classified as telolecithal. 

THE EGG OF THE CHICK. The hen's egg (Fig. IOC) is ex- 
tremely telolecithal. The cytoplasm, with the nucleus in its 
center, forms a small germinal disc upon the great mass of yolk. 

This yolk is arranged in 
concentric layers of 
yellow and white mate- 
rial around a central 
mass of white yolk, 
called the latebra (Fig. 
11). From this latebra 
a stalk of white yolk (the 
neck of the latebra) ex- 
tends upward. The 
germinal disc rests on 
this isthmus. The yolk 

and germinal disc are surrounded by a delicate vitelline membrane 
(primary). This in turn is surrounded by the albumen, a viscous 
tertiary membrane twisted spirally about the egg from left to 
right, starting from the broad end of the egg. The albumen 
next to the vitelline membrane is denser than the rest and is 
prolonged into two spirally twisted cords, the chalazae, one 
at either end of the egg. The albumen is in turn surrounded 
by two parchment-like shell membranes, of which the inner 
one is the thinner. These two are separated at the blunt end 
of the egg, thus forming the air chamber. The egg shell is a 
calcareous deposit upon the outer shell membrane. Its color is 
due to bile pigments of the hen. The germinal disc is about 
4 mm. in diameter, the yolk about 40 mm. The size of the egg 
as a whole varies largely, depending on the amount of albumen 
deposited around the yolk. 

Giant and dwarf eggs are sometimes recorded. In the hen's 
egg, double- and triple-yolked eggs are known, as well as those 
which have no yolk at all. A very strange abnormality is known 
as the " ovum in ovo," where one egg is formed around another. 
The eggs of birds are either male-producing or female-producing, 
a statement based solely on the evidence of genetics as no visible 
differences have been observed. 





THE EGG OF MAN. The human egg (Fig. 10D) is extremely 
smalj. The yolk granules are concentrated about the nucleus, 
which is slightly excentric. It is not positively known whether 
a vitelline membrane is present. But the egg is enclosed in a 
thick capsule with radial striatiorjs (canals?), the zona pellucida. 
It is not clear whether this is a primary or secondary envelope. 
At the time the egg leaves the ovary it is still surrounded by a 
layer of follicle cells which 

make up the so-called ^ \^ ^f,^ ,zona P ciiucida 

corona radiata (Fig. 12). 
The egg may be termed 
isolecithal. Its diameter 
is about 0.13 mm. 

Eggs and their environ- 
ment. Needham has 
recently pointed out that 
eggs differ from one an- 
other in respect to the 
physico-chemical consti- 
tution of the unfertilized 
egg, and the possibility 
of obtaining necessary material from the environment. The 
marine egg, exemplified by the amphioxus, develops in a medium 
containing oxygen and inorganic salts. The egg is organized in 
such a manner as to facilitate the exchange of materials with the 
environment, and the yolk is small in amount and (to judge from 
analyses made on marine fish) relatively poor in fats and inorganic 
salts. Development is rapid up to the hatching stage, but there- 
after the larva takes a long time to attain its full size and sexual 

The egg which develops in fresh water, like that of the frog, does 
not have a medium so rich in salts as the marine egg. It is there- 
fore originally equipped with a larger store of this material. But 
the aqueous medium still affords facilities for the exchange of 
carbon dioxide and oxygen and for the disposal of nitrogenous 
wastes. The jelly with which the frog's egg is provided consists 
almost entirely of protein and water. Diffusion takes place 
through it readily, and it affords protection against mechanical 


12. Human egg (ovarian) X200 (after 


injury and bacterial infection, as well as furnishing a source of 
nourishment immediately after hatching. 

The terrestrial (cleidoic) egg, such as that of the hen, stands 
easily first in respect to the amount of yolk present. The ratio 
of fat to protein in the yolk is also the highest. It is obvious that 
the egg must contain all the material necessary for growth except 
free oxygen and water, for these are the only substances passing 
from the atmosphere through the protective envelopes of the egg. 
Hence, as pointed out by Milnes-Marshall, except in the earliest 
stages the chick develops more rapidly than the amphioxus and 
attains its adult form in a much shorter time. The egg albumen 
also a source of food is a watery solution of protein with some 
carbohydrates. As we shall see in later chapters, the relative 
isolation of the embryo in the cleidoic egg is correlated with the 
development of its extra-embryonic sacs, i.e., the amnion or water 
bath, and the allantois which serves in the first instance to store 
nitrogenous wastes. 

The uterine egg, typical of the mammals, is characterized by 
little yolk, for, from a very early period, its nourishment is derived 
exclusively from the body of the mother. Accordingly there is a 
precocious separation of a special layer, the trophoblast, con- 
cerned with implantation, and later the development of a special 
organ of interchange, the placenta. 

Comparison of the egg and the sperm. Both gametes are 
morphologically complete cells. Each has a nucleus and a cyto- 
some containing representatives of the centrosomes, mitochondria, 
and Golgi bodies. Each has a plasma membrane. Yet neither is 
capable of independent, continued existence, for physiologically 
they are unbalanced. The egg is large, inert, and contains a vast 
store of metaplasm, is protected by egg envelopes, and has lost 
the power of continued division. The sperm is small, highly 
motile, contains little cytoplasm and no metaplasm, is devoid of 
protective membranes, and in itself has lost the power of con- 
tinued division. We shall now turn to the study of the develop- 
ment of the germ cells and see how the structural differences, at 
least, arise. 





Ceil structures 


Large amount 


Small amount 

One, disappears in maturation 


Two, retained in maturation 



Spiral coil 


Golgi bodies 



Plasma membrane 









Nuclear membrane 


Other differences 






Swims actively 


Motile organs 


Egg envelopes 






Few to many 

Numbers produced 

Very many 


Gametogenesis is the term applied to the history of the gametes 
their origin and development (Fig. 13). The special history 
of the male gametes is called spermatogenesis, that of the female 
gametes oogenesis. 

The origin of the germ cells. Weismann is responsible for a 
theory that the germ cells separate completely from all the other 
cells of the body (soma cells) at a very early stage in development. 
There is some evidence for this in the embryology of a few in- 
vertebrate animals such as Ascaris, a parasitic roundworm. In 
the very first cleavage of the fertilized egg, the two daughter cells 
show a .striking difference, for when one of the daughter cells 
divides it retains all the chromatin of its nucleus whereas the other 
gives up a portion of this material to the cytosome. This phe- 
nomenon has been called chromatin diminution, and the cell 
showing this characteristic becomes a soma cell. The other is 




Germ Cells 





FIG. 13. Diagram of gametogenesis, male on left, female on right (after Wilson). 



known as a stem cell (Fig. 14), and in its division it produces in 
turn one cell which will be a soma cell and one which will be a 
germ cell. Eventually a stem cell gives rise to two identical 
cells, both of which are germ cells. These are known as primor- 

S 2 (EMSt) 



FIG. 14. Origin of stem cells in Ascaria. A, first cleavage. B, C and D, second 
cleavage. PI and Pj are stem cells. Si (which gives rise to A and B) and 82 are 
soma cells. (From Richards after Boveri.) 

dial germ cells, and, from this time on, they and their descendants 
produce germ cells only. 

This theory of the distinction between germ cells and soma 
cells has held an important place in the history of biology because 


it seemed to deny the possibility of the inheritance of character- 
istics acquired after fertilization. In other words, the^character- 
istics would be acquired by the soma cells whereas inheritance is 
transmitted by the germ'cefls which are entirely distinct. Now 
that we know TEaFTEe' nuclei of all cells are identical, whether 
they are germinal or somatic, the theory of the continuity of the 
germ cells has less theoretical importance. 

Primordial germ cells. In all vertebrates, so far as is known, 
the germ cells are first recognizable in the lining of the gut at a 
very early stage of development. These primordial germ cells, 
as they are called, are distinguishable by their large size, clear 
cytoplasm, and heavily staining nucleus (Fig. 15). From the gut 

Mesonephric duct 

Postcaval vein 

germ cell 

Dorsal mesentery 

FIG. 15. Primordial germ cells in the frog (Rana sylvatica). Part of transverse 
section through 10 mm. larva, showing coelomic roof, X375. (After Witschi, 

wall they migrate into the mesentery suspending the gut from the 
roof of the coelom, and thence to the wall of the coelom. Here 
they multiply rapidly and produce two longitudinal ridges, which 
are the primordia of the sex glands, or gonads. 

The 'gonia. There ar two opinions concerning the fate of 
the primordial germ cells ih vertebrates: one that they give rise 
to all the later generations of germ cells; the other that they 
degenerate and the later germ cells arise independently from the 
tissue of the gonads. In any case, the germ cells which continue 
to multiply actively in the gonads are known as 'gonia : sperma- 
togonia if they are to give rise to sperm, oogonia iKEey j^v 
to eggs. '" ' ' '"" "' "' * ' 





The 'cytes. When the individual becomes sexually mature, 
individual 'gonia undergo a period of growth by means of which 
they become * transformed into 'cytes (auxocytes, meiocytes): 
spermatocytes if male, oocytes if female. The 'cyte (Fig. 16) is 
a large cell with a vesicular nucleus, two centrosomes surrounded 
by a clear area sometimes known as the sphere substance, which 
is in turn surrounded by a layer 
of Golgi bodies, and a cloud of 

The maturation divisions. 
Each 'cyte gives rise to four 
daughter cells or gones (Sharp) by 
means of two cell divisions. 
These divisions are unique because FlG 16 _ Dia ~ of 
of certain internal phenomena and 
are known as the maturation 

divisions. The nature of these divisions will be discussed in more 
detail in a later chapter (page 64). Meantime we note that the 
spermatocyte gives rise to four cells of equal size, the spermatids, 
each of which will be transformed into a sperm. The oocyte on 
the contrary gives rise, by the first maturation division, to two 
cells one of which is very minute, the first polar body (polocyte I). 
The larger cell undergoes a second unequal division, resulting in 
the production of a second polar body (polocyte II) and the mature 
egg or ovum. It will be recalled that among the vertebrates the 



Golgi bodies 

an ^ , cyte 
(After Wilson.) 






Period of metamorphosis 

(Stem cells) 
Primordial germ cells 
Period of migration 

Period of multiplication 

Period of maturation 

Gones (Sharp) 

Ovum and polocytes (06tids) 



sperm enters the egg before the production of the second polar 
body. Sometimes the first polar body also divides so that four 
cells (ootids) may be produced by the oocyte. 

Spermatogenesis. The male 'cyte (primary spermatocyte) is 
a large cell containing a large vesicular nucleus, more or less 
excentric. Near the nucleus, and in the center of the thicker 
layer of cytoplasm surrounding it, are to be seen one or two 
centrosomes, surrounded by a clear substance known as the sphere 
substance. This compound body is known as the idiosome, and 
with it are often associated the Golgi bodies, sometimes so closely 
connected as to form an investing reticulum or even a shell. 
Around the idiosome are also grouped the mitochondria forming 
a cap which sometimes includes the nucleus as well. 

The primary spermatocyte divides, giving rise to two secondary 
spermatocytes, which divide again, often without intermission, 
each forming two spermatids. The four spermatids thus pro- 
duced irom the primary spermatocyte are later transformed into 
the sperms. 

During the two divisions mentioned above, the chromatin of 
the spermatocyte nucleus is distributed to the spermatids in such 
a way that they will differ from each other in respect to the 
nuclear contents. The details will be discussed later (page 64). 
The centrosomes divide at each cell division so that each sperma- 
tid has a centrosome. The Golgi bodies, each with a small 
amount of sphere substance, are divided among the four sperma- 
tids, in each of which they aggregate to form an idiosome. The 
mitochondria are divided with almost exact evenness among the 
spermatids, in each of which they assemble to form a paranucleus 
(nebenkern). A plasma membrane is present. 

In the transformation of the spermatid into the mature sperm 
(Fig. 17), the nucleus, having previously extruded a large amount 
of material, condenses into a deeply staining mass which elongates 
into its final shape, The centrosome divides, and the two centro- 
somes take up a position which marks the posterior end of the 
future sperm, one centrosome (proximal) lying against the 
nucleus, the other (distal) posterior to the first. The paranucleus 
also takes a posterior position while the idiosome moves around 
the nucleus to the anterior end. The greater part of the cyto- 
plasm is sloughed off. 



Wall of ovary 

Follicle cells 



FIG. 19. 

Transverse section through part of 
frog ovary. X95. 

Ovulation. Within the ovary the vertebrate egg is sur- 
rounded by nurse cells which make up a nest or follicle (Fig. 19). 
Within this it enlarges and may undergo its first maturation 
division. Periodically, varying from once a year in most verte- 
brates to once a month in the human species, or daily in the 
domestic fowl, eggs are dis- 
charged from the ovary. In 
numbers this discharge 
varies from a single egg as 
in man or the fowl to 
thousands in the frog or 
millions in many fish. 

The factors bringing 
about ovulation are diverse. 
In the frog it has been 
shown by Hugh 1 that ovula- 
tion is brought about by 
the contraction of a thin 
muscular layer in each fol- 
licle, plus the action of an 
enzyme which digests the outer wall of each follicle and thereby 
weakens it. In mammals a follicular fluid is secreted about 
the egg, enlarging the follicle until it protrudes from the sur- 
face. Finally the outer wall of the follicle, now very thin, 
ruptures, owing perhaps to factors similar to those acting on the 
frog's egg. It has been shown in many vertebrates that ovulation 
can be induced at any time by means of a hormone secreted by 
the anterior lobe of the pituitary gland (page 332). 

From the ovary the eggs are caught up in the open end of the 
oviduct, down which they pass to the exterior. In many aquatic 
forms they are discharged directly. In others they accumulate 
in an enlarged portion of the oviduct known as the uterus, await- 
ing discharge from the body; such animals are known as oviparous 
(amphioxus, frog, chick). In still others the egg remains in the 
uterus until development has reached an advanced stage; these 
are the viviparous animals (man, etc.). 

Semination. This term is applied to the discharge of the 
sperms. These cells remain in the testis (Fig. 20) until mature, 

1 Jour. Exp. Zool. In press. 



often attached to nurse cells. When discharged they pass through 
tubules of the testis which lead directly to a sperm duct. They 
become motile upon reaching the medium in which fertilization 
takes place. Enormous numbers are produced at a single dis- 
charge (over 200,000,000 in man). 

In aquatic animals such as the amphioxus and fish the two 
sexes congregate together at the breeding season, and eggs and 
sperms are discharged together. In some cases even aquatic 
animals have copulatory organs which introduce the sperms into 
the oviduct, bringing about internal fertilization. In the frog, 






Fia. 20. Section through part of frog testis. X200. 

the males and females unite in pairs (amplexus), thus ensuring 
that the sperms are discharged simultaneously with the eggs so 
that fertilization, although external, is regulated. In all ter- 
restrial vertebrates fertilization is internal. 

Fertilization. The actual fertilization of the egg (syngamy) 
has been observed in the amphioxus and the frog, but our detailed 
knowledge of the process is obtained from the study of such 
marine invertebrates as the sea urchin. The essential act in fer- 
tilization is the entrance of a single sperm into the egg and the 
coming together of the two nuclei (pronuclei) (Fig. 21). But this 
phenomenon is preceded by other events concerned in bringing 
the sperm to the egg. 

Attraction. One of the factors believed to bring the sperm 
towards the egg is an attraction (chemotaxis) caused by the emis- 
sion of some chemical substance by the egg or the female sex 



organs. It is known also that sperms swim in a spiral path, and 
it has been suggested that when they come in contact with a solid 
object they remain in contact with it (thigmo taxis). If the spiral 
brings a sperm obliquely towards the egg the contact flattens out 
the spiral, causing the sperm to remain in contact without pene- 
tration. But if the sperm arrives at the egg in a radial direction 
penetration is facilitated. Lillie has shown that the sea-urchin 
egg emits a secretion (fertilizin), which brings about a temporary 
and reversible adhesion of the sperm heads in clusters (agglutina- 
tion). Fertilizin is produced only after the egg is mature and 
before it is fertilized. 

Penetration. The sperm bores its way through the egg 
envelope but then apparently comes to rest against the plasma 

O Copulation path 


FIG. 21. Diagram to show fertilization of the egg. A, fertilization cone. B, pene- 
tration path. C, female copulation path, and rotation of sperm head. D, male 
copulation path. E, cleavage path. F, first cleavage. 

membrane. Meantime there appears at the surface of the egg a 
cone or even a long filament of cytoplasm which comes in contact 
with the sperm head. It then retracts drawing the sperm to the 
egg and engulfing it (Fig. 21 A, B). Thereafter, and commencing 
at the point where the sperm head was engulfed, a thin layer of 
surface protoplasm is elevated to form what is known as a fertil- 
ization membrane (vitelline membrane?). In older days, when 
it was thought that the sperm bored its way into the egg, it was 
believed also that the fertilization membrane acted as a bar to 


other sperms. Apparently the elevation of the membrane is due 
to the secretion of some fluid from the egg, which decreases in 
diameter at the same time. Okkelberg describes a loss of 14 per 
cent in the volume of the egg of the brook lamprey. The forma- 
tion of this membrane with its perivitelline fluid underneath 
marks the successful fertilization of the egg. For example, the 
fertilized frog's egg will rotate within this membrane. 

The pronuclei. After the second maturation division, which 
does not take place in vertebrates until after the entrance of the 
sperm, the nucleus of the ovum (female pronucleus) is near the 
periphery at the animal pole, while the nucleus of the sperm 
(male pronucleus) is at the periphery near the point of penetra- 
tion. The sperm head rotates 180 so that the male pronucleus 
now lies distal to the middle-piece containing the centrosome 
(Fig. 21C). The two pronuclei come together (Fig. 21B, C) by 
a route which may be analyzed into the following components: 
(1) the sperm penetration path, which is usually the radius of the 
egg at which the sperm entered; (2) the sperm copulation path, 
which is directed towards the point at which the pronuclei will 
meet and is often at a considerable angle to the penetration path; 
(3) the egg copulation path, along which the female pronucleus 
moves towards the meeting point; and (4) the cleavage path 
(Fig. 21E), along which the two pronuclei move to their final 
position on the egg axis, often slightly nearer the animal pole. 
The two pronuclei may unite to form a common reticulum, or 
they may remain close together contributing independently to 
the first division of the zygote (Fig. 21F). See also page 156. 

The centrosome of the egg disappears after the second matura- 
tion division. The centrosome of the zygote, therefore, is either 
the centrosome of the sperm or, as it is believed in some cases, a 
new one developed in the egg cytoplasm near the engulfed sperm 

The mitochondrial material of the sperm fragments and is dis- 
tributed throughout the cytoplasm of the zygote. The later 
history of the acrosome has not been followed. 

There is much divergence among different kinds of animals 
with respect to those parts of the sperm which are actually 
engulfed in the egg. In some, it is the entire sperm; in others, 
only the sperm head. 



(5) Ground 


Presumptive organ regions. In many different kinds of eggs, 
the student of cellular embryology has been able to recognize 
different regions by differences in the cytoplasm, such as the 
presence or absence of pigment, mitochondria, yolk, etc., and to 
trace the distributions of these materials into the different 
daughter cells as cleavage takes place. These presumptive organ 
regions, as they may be called, are usually more easily demon- 
strated after fertilization. For example, before fertilization the 
living egg of the tunicate Styela (Cynthia), according to Conklin 
(1905), has orange pigment 
granules uniformly distributed 
in its outer layer of cytoplasm. 
During fertilization, following 
an intricate series of stream 
movements, the orange pigment 
(Fig. 22) is concentrated in a (2)Gwy 
crescentic area at what will later 
be the posterior surface. Im- 
mediately above this is a similar 
area of clear protoplasm. On 
the opposite side of the egg at 
what will become the anterior 
surface is a gray crescent. Be- 
low these crescents the vegetal 
hemisphere is marked by the 
presence of gray yolk. In later 
cleavage, the yellow crescent will be distributed to the cells which 
form the mesoderm, the gray crescent to the cells which form the 
notochord and neural plate, the gray yolk to the cells of the en- 
doderm, and the remainder of the egg goes to the cells of the 
epidermis. The materials contained in the different presumptive 
organ regions are frequently called organ-forming substances. 
The regions themselves are called presumptive endoderm, pre- 
sumptive mesoderm, etc. 

Parthenogenesis. This term is applied to the development 
of a new individual from an unfertilized egg. It does not occur 
naturally among the vertebrates but may be illustrated by the 
honey bee, in which the unfertilized egg develops into a male 

(3) Grey yolk 

Fia. 22. Presumptive organ regions 
(organ-forming substances) in egg of 
Styela after fertilization, viewed from 
left side, approx. X250. (After Conk- 



(drone) and the fertilized egg becomes a female (either queen or 
worker according to the type of food supplied). 

Artificial parthenogenesis has been produced in the frog's egg 
by slight punctures with a finely pointed glass needle. Most of 
the parthenogenetic eggs do not go far in development, but Loeb 
was able to raise, out of many thousands of treated eggs, a few 
adult frogs (15 males, 3 females, 2 doubtful). 

FERTILIZATION OF THE AMPHioxus EGG. In the case of the 
amphioxus, fertilization is external. The males and females leave 
the sands to swarm in the shallow waters during late afternoons 
of spring and summer. Eggs and sperms are discharged, from 
the segmental gonads in which they develop, into the cavity 
of the atrium, and escape to the exterior through the atriopore. 
The first polocyte is given off before fertilization. Immediately 
after fertilization the vitelline, now the fertilization, membrane 


2nd Polocyte 







FIG. 23. Presumptive organ regions in egg of the amphioxus, one hour after fer- 
tilization. Sagittal section, approx. X220. (After Conklin 1932.) 

expands greatly, leaving the egg in a large perivitelline space 
(Figs. 23 and 10A). The second polocyte given off after the for- 
mation of the fertilization membrane remains attached to the 
egg while the first is usually lost to view. 

The fertilized egg of the amphioxus (Conklin 1932) shows in 
sections (Fig. 23) a crescent of more deeply staining protoplasm 
on the side of the egg which will give rise to the posterior part of 
the body. This crescent will form the cells of the mesoderm 
(compare the orange crescent of Styela). Opposite this is a less 
clearly defined crescentic area from which the cells of the noto- 
chord and neural plate will be formed (compare the gray crescent 



of Styela and of the frog). The material of the vegetal hemi- 
sphere, bounded above by these crescents, will form the endo- 
derm; the material of the animal hemisphere above the crescents 
will form the epidermis. 

FERTILIZATION OF THE FROG'S EGG. The fertilization of the 
frog's egg is external, but the sperm are brought into close prox- 
imity to the eggs during the sexual embrace or amplexus. During 
the breeding season the males embrace the females with the 
fore-legs, at which time the germ cells of each are extruded. The 
sperms make their way through the egg jelly before this envelope 








FIG. 24. Diagram of frog's egg after fertilization to show gray crescent. The line 
immediately external to the vitelline membrane and polocytes represents the 
"chorion." XlO. 

has attained its final thickness. The entire sperm enters the egg 
usually within 40 of the apical pole. The vitelline membrane 
is thrown off as the fertilization membrane, leaving a perivitelline 
space within which the egg may rotate. The second maturation 
division then occurs, followed by the conjugation of the pronuclei. 
The penetration and sperm copulation paths are marked by a 
trail of pigment dragged in with the sperm head. A single sperm 
enters the egg. Immediately upon fertilization the cortical cyto- 
plasm of the egg rushes towards the point of penetration, carrying 
with it the black pigment (melanin) of the animal hemisphere. 
Upon the side of the egg opposite the point of penetration there 
appears a crescent-shaped area in which the pigment is less dense 



and which is therefore known as the gray crescent (Fig. 24). 
The region gives rise to the notochord and neural plate. 

FERTILIZATION OF THE HEN'S EGG. In the fowl, fertilization 
is internal. The sperms, introduced into the cloaca of the female 
during copulation, make their way to the upper end of the oviduct, 
where fertilization takes place. Five or six sperms enter the 
germinal disc, where they remain inactive until after the second 
maturation division. One of them then moves inward until it 
comes in contact with the female pronucleus, which has itself 
moved downward from the surface of the germinal disc. The 
supernumerary sperms move outward to the border of the disc, 
where, after a few divisions, they degenerate. The fertilized egg 
moves slowly down the oviduct while the tertiary envelopes are 
forming about it. 

FERTILIZATION OF f HE HUMAN EGG. Fertilization is internal 
and occurs at the upper end of the oviduct (Fallopian tube). It 







FIG. 25. Fertilization of the guinea pig egg. Three stages following that shown 
in Fig. 20A. (After Lams.) 

is probable that a single sperm enters the egg after the first 
maturation division. Further details are lacking, as no direct 
observations have been recorded. Figure 25 illustrates fertiliza- 
tion in the egg of the guinea pig. 


The gametes are atypical cells, the egg and sperm differing both 
from each other and from a composite cell. The egg most re- 
sembles a composite cell, from which it differs in the absence of a 


centrosome after it has become mature. It is large, quiescent, 
and protected by envelopes. The sperm, almost devoid of cyto- 
plasm, is small, active, and naked. 

The gametes are derived from primordial germ cells which are 
first recognizable in the wall of the gut. Thence they migrate 
into the roof of the coelom where they multiply rapidly giving rise 
to the gonad. In the gonad multiplication continues until, when 
the individual is attaining maturity, some of the 'gonia enlarge 
to become 'cytes, each of which will undergo two meiotic divisions. 
The spermatocyte gives rise to four spermatids each of which will 
be transformed into a sperm. The oocyte, on the contrary, gives 
rise to an ovum and two or three polocytes. 

The zygote, or fertilized egg, arises from the union of an egg 
and a sperm. This union is preceded by the discharge of eggs 
from the ovary (ovulation) and sperms from the testis (semina- 
tion). The sperm, attracted to the egg, enters it due to the 
mutual action of the two gametes, and the nuclei of the two 
gametes come together, each to contribute to the first division of 
the fertilized egg. 

After fertilization, and sometimes even before, it can be seen 
that the egg has a definite organization, manifest in its polarity 
(seen even in ovarian eggs) and, in especially favorable material, 
evident in presumptive organ regions. 


^llen, E. (ed.) 1932. Sex and Internal Secretions, Chap. 14. 

Brachet, A. 1921. Traite" d'embryologie, Book 1. 

Cowdry, E. V. (ed.) 1924. General Cytology, Sections I, V-VIII. 

Hertwig, O. (ed.) 1906. Handbuch, etc., I, Chaps. 1 and 2. 

Jenkinson, J. W. 1913. Vertebrate Embryology, Chaps. 3 and 4. 

Kellicott, W. E. 1913. General Embryology, Chaps. 2 to 5. 

Kerr, J. G. 1919. Textbook of Embryology, I, Chap. 1. 

iCorschelt, E., and Heider, K. 1902. Lehrbuch, etc. Chaps. 4 to 6. 

iallie, F. R. 1919. Problems of Fertilization. 

tx>eb, J. 1916. The Organism as a Whole from a Physicochemical Viewpoint. 

Meisenheimer, J. 1921. Geschlecht und Geschlechter im Tierreiche. 

Morgan, T. H. 1927. Experimental Embryology. 

Wilson, E. B. 1925. The Cell, etc. Chaps. 1-6, 9-12. 

Sharp, L. W. 1934, Introduction to Cytology, Chaps. 1-16. 


The germ cells are really cells detached from the bodies of the 
parents. When they unite in fertilization they bring together 
material from both parents. Herein lies the explanation of the 
inheritance of parental characteristics, of the fact that the fertil- 
ized egg develops in a way characteristic of the species and the 
fact that individuals differ from one another. In the following 
paragraphs we shall review the theory that the individual units 
of heredity are the genes, borne in the chromosomes, distributed 
in the maturation divisions, and brought together in fertilization. 


It will be necessary first to describe the chromosomes as they 
behave in ordinary (somatic) cell division, then to point out the 
peculiar features of the maturation (meiotic) divisions and of 
fertilization, and finally to indicate how this behavior of the 
chromosomes fits the known laws of heredity. 

The chromosomes in mitosis. The division of most cells is 
accompanied by the formation and longitudinal division of threads 
of chromatin, called chromosomes, in the nucleus. This type of 
cell division is known as mitosis (Fig. 26). Some cells, however, 
divide without the formation of chromosomes (amitosis), and the 
daughter cells are thereafter incapable of mitotic division. For 
the sake of convenience we may use the terms karyokinesis for 
the division of the nucleus in mitosis and cytokinesis for the 
division of the cytosome. 

Karyokinesis. Before cell division the metabolic (" resting ") 
nucleus is a reticulum of chromatin lying in the fluid karyolymph 
with a nucleolus, the whole surrounded with a nuclear membrane 
(Fig. 26A). In mitosis we distinguish four stages, prophase, 
metaphase, anaphase, and telophase. 

In the prophase the reticulum separates into its constituent 
threads, chromonemata, by the breaking down of the smaller 




threads connecting them. Very early it can be seen that these 
threads are double or split longitudinally (Fig. 26B). Soon 
thereafter a matrix is visible about the two chromonemata. 
This compound structure, consisting of the two chromonemata 


FIG. 26. Diagrams of somatic mitosis. A, metabolic ("resting") stage. B, early 
prophase showing chromonemata and attachment points. C, middle prophase, 
matrix appearing. D, late prophase, chromonemata obscured. E, metaphase. 
F, anaphase. G, early telophase, matrix disappearing. H, middle telophase, 
nuclear membrane forming. I, late telophase, reticulum developing. (Based on 
a diagram by Sharp.) 

and the surrounding matrix, is a chromosome (Fig. 26C). The 
number of chromosomes so formed is the same in every cell of 
every individual belonging to any particular species. (This 
statement is subject to exceptions. See pages 69 ff.) Towards 
the end of the prophase the chromonemata are usually invisible. 


Finally the nuclear membrane disappears, and the karyolymph 
assumes the form of a double cone or spindle (Fig. 26D). 

In the metaphase (Fig. 26E), the chromosomes line up in an 
equatorial plane through the spindle. Each has a definite at- 
tachment region lying in the equatorial plane even though the 
ends of the chromosomes may lie outside of the plane. 

In the anaphase (Fig. 26F), the chromosomes separate into two 
longitudinal portions each containing one of the original chro- 
monemata with surrounding matrix. Preceded always by its 
attachment region each daughter chromosome moves towards a 
pole of the spindle. Carothers (1934) describes the growth of a 
fiber from the attachment region of each daughter chromosome 
to the nearest pole of the spindle. Eventually two equivalent 
sets of chromosomes are formed, one in the vicinity of either pole, 
each set containing a daughter chromosome from each of the 
original chromosomes formed in the prophase. 

In the telophase (Fig. 26G, H, I), each set of chromosomes as- 
sumes the metabolic condition. The matrix loses its staining 
capacity and the chromonemata reappear, often already split 
longitudinally. The nuclear membrane is formed about each 
group, the chromonemata are united by tiny cross-strands, the 
nucleolus reappears, and the nucleus is seen to be filled with 
karyolymph. The cell now contains two daughter nuclei each 
identical with the other and with the parent nucleus. 

Cytokinesis. Other striking events are taking place in the 
cytosome during mitosis. During the prophase the centrosome, 
if not already divided, separates into two daughter centrosomes 
which move apart. About each of them is a spherical mass of 
protoplasm, often containing radial striations, known as the aster. 
Between them is a central spindle apparently containing fibers. 
Cytologists distinguish three types of fibers: (1) primary or con- 
tinuous fibers extending from centrosome to centrosome, (2) half 
spindle components extending from chromosome to centrosome, 
and (3) interzonal connections extending between the separating 
daughter chromosomes (Schrader). The centrosomes reach the 
opposite sides of the nucleus just as the nuclear membrane dis- 
appears. The karyolymph apparently unites with the material 
between the two centrosomes to form the mitotic spindle along 
which the chromosomes move in the anaphase. In the telophase, 



asters and spindle disappear and the centrosomes alone remain in 
the positions they occupied at the poles of the mitotic spindle* 
Sometimes they divide in anticipation of the next mitosis. 

The mitochondria usually divide en masse (Fig. 27 A). This 
division of the mitochondria is approximately an equal one, and 
there is some reason to believe that the individual mitochondria 
divide during mitosis or just prior to it. 

The Golgi bodies, even when aggregated into a Golgi apparatus, 
separate during mitosis and are segregated into the daughter cells, 
usually associating themselves with the two centrosomes (Fig. 
27B). It is uncertain whether each Golgi body divides individ- 




Golgi bodies 

FIG. 27. The mitochondria and Golgi bodies in mitosis. 
B, GoJgi bodies. (After Bowen.) 

A, mitochondria. 

ually at mitosis, but some evidence has been brought forward to 
support this contention. 

In animal cells the cytosome as a whole divides by construction. 
In this process the cell elongates in the direction of the spindle 
during the anaphase and telophase. Following the reconstruction 
of the daughter nuclei in the telophase, a furrow appears at the 
periphery of the cell, around the equatorial belt, and at right 
angles to the axis of elongation. This furrow advances towards 
the center of the cell until the cell is completely divided. 

Distribution of the chromosomes. Each daughter cell has 
approximately half of the cytoplasm proper, half of the mito- 
chondria and Golgi bodies, a centrosome derived from that of the 


parent cell, and a nucleus built up from a set of chromosomes, 
each of which was produced by the division of a chromosome in 
the parent cell. It is apparent from the foregoing account that 
the key to the complexities of mitosis is the division of the chromo- 
somes. The achromatic figure is the framework upon which this 
division takes place. The division of the mitochondria and Golgi 
bodies is still too little understood. But the chromosomes, ap- 
pearing in the prophase, halved with such accuracy in metaphase 
and anaphase, and disappearing again in the telophase, are charac- 
terized by a constancy in number, an individuality evinced in 
form and behavior, and a persistence from generation to genera- 
tion. In some favorable material it has even been possible to 
demonstrate that the chromonemata arise in the prophase exactly 
as they merged into a reticulum in the previous telophase. From 
the statements above, it is not unreasonable to draw the conclu- 
sion that the chromosomes are directly concerned with inheritance 
in cell reproduction. 

The chromosomes in meiosis. During the two maturation 
divisions by which the gametes are formed, the number of chromo- 
somes is reduced to one-half the number characteristic of the 
species. Since in the ordinary somatic mitosis the number of 
chromosomes given to each daughter cell is exactly the same as 
that of the parent, it is evident that we are dealing with a peculiar 
type of mitosis (Fig. 28). The name meiosis is frequently applied 
to the maturation divisions. 

First meiotic division. The essential feature in which the 
first meiotic division differs from the ordinary (somatic) mitosis 
is that during the prophase the chromosomes unite in pairs 
(Fig. 29, 2). This is synapsis and occurs only in the first meiotic 
division. Since eacF of tKe Chromosomes always divides during 
the prophase also (Fig. 29, 4), the net result is that at the end of 
the prophase there are only half the number of chromosomes seen 
in somatic mitosis, but each of these consists of four parts (chro- 
matids) instead of two (Fig. 29, 5). These compound bodies con- 
sisting of four chromatids are called tetrads (Fig. 29, 6). The 
quadripartite nature of the tetrad may be expressed by the 

A * a 
formula . , where A represents one of the synaptic mates and a 

the other. 

ABC ^ be 

ABC a be 




Firs* column i 
division of a diploicf 
chromosome complement. 

( Second column? The meiotfc 
divisions, chan0in3 tbc dip'oid 
to the monoploid 




Fia. 28. Comparison of somatic and meiotic mitosis. (From Sharp.) (66) 



FIG. 29. Diagram of meiosis. 1, first meiotic division, prophase, (leptonema 
stage). #, do. showing synapsis (zygonema stage). S } do. showing thickening of 
the chromosomes (pachynema stage). 4, do. showing forma tion of tetrads (diplo- 
nema stage). 5, do. showing condensation of matrix (diakinesis stage). 6, meta- 
phase I. 7, anaphase I showing dyads. 8, telophase I. 9, second meiotic 
division, prophase showing dyads united at attachment points, 10, metaphase II. 
11, anaphase II showing the separation of the chromatids which composed the 
dyads. 12, telophase II. Each of the four germ cells now has the haploid num- 
ber of chromatids (chromosomes). (From Sharp.) 


In the anaphase (Fig. 29, 7), the daughter chromosomes each 
possess two chromatids and are known as dyads. But there are 
two different ways of dividing a tetrad. In one case the, two 

chromatids derived from one of the synaptic mates ( -r J might be 

separated from those derived from the other mate ( - ) in a reduc- 
tion (disjunction) division. In the other, each dyad might con- 
tain one chromatid from each of the synaptic mates (A : a) as the 
result of an equation division. 

The telophase (and prophase of the second meiotic division) 
sometimes is omitted if the second division succeeds the first 

Second meiotic division. If these omissions take place, each 
of the daughter 'cytes divides immediately, the chromosomes, 
still in the dyad condition, lining up on the spindles for the meta- 
phase of the second meiotic division. But even if the telophase 
of the first and prophase of the second meiotic divisions are not 
omitted (Fig. 29, 8), it is obvious that the chromosomes arising in 
the prophase (Fig. 29, 9) are dyads and that they undergo no 
other longitudinal split. The anaphase of the second meiotic 
division (Fig. 29, 11) merely separates the two chromatids of each 
dyad from each other. The final result is that the four cells pro- 
duced by the meiotic divisions (Fig. 29, 12) each have one chroma-^ 
tid from each tetrad or one-half the number of chromosomes found 
before meiosis took place. This is expressed in another way by 
saying that the number of chromosomes has been reduced from 
the diploid to the haploid (monoploid) number. 

Here we must note that ft makes no difference whether the first 
meiotic division divided a tetrad reductionally or equationally. 
The second division always distributes the two chromatids of each 
dyad into different cells. Each of the four daughter cells has onei 
chromatid from each tetrad, and therefore one representative fromj 
either one of the two synaptic mates (A or a), but not from bothj 

Distribution of the chromosomes. As each tetrad orients 
itself independently upon the spindle it is evident that it is a 
matter of chance which half of a tetrad, or of a dyad, goes to 
either pole of the spindle. Accordingly, if we had eight chromo- 
somes, A, a, B, &, C, c, D, and d, these would unite in synapsis to 



form four double chromosomes, Aa, Bb, Cc, and Dd. These 
^ r . . , A:a B:b C:c A D:d _ _ 
would form the four tetrads, ^~r^> IT-T' CT-'c' a ZT 7 ^' 

lowing the two meiotic divisions (equation and reduction, regard- 
less of their order), the mature germ cells would have four 
chromosomes (the haploid number), but only one representative 
of each synaptic pair. The possible combinations are 2 4 or 16, 
namely, ABCD, ABCd, ABcd, Abed, ABcD, AbCD, AbCd, 
aBCD, aBCd, aBcd, abed, aBcD, abcD, abCD, and abCd (Fig. 30). 

FIG. 30. Showing the distribution of the chromosomes in fertilization and the 
following meiotic divisions. (After Wilson.) 

Accordingly the number of different types of gametes which may 
be formed can be determined from the formula 2 n when n is the 
haploid number of chromosomes characteristic of the species. 

The chromosomes in fertilization. Evidently when the egg 
and sperm unite in fertilization, the pronucleus contributed by 
each contains the haploid number of chromosomes. In this way 
the diploid number characteristic of the species is restored. It is 
obvious that, unless the number had been reduced by meiosis, it 
would be doubled in each new generation. 

In the second place, it is clear that each germ cell contributes a 
homologous set of chromosomes, and that in synapsis the chromo- 
somes unite in homologous pairs. In the example referred to 


above, chromosomes A, J3, C, D came from one parent and a, &, c, 
rf came from the other. We can now visualize each synaptic pair 
as consisting of one paternal and one maternal chromosome. 

During meiosis the paternal and maternal chromosomes are 
sorted out into different assortments in the different germ cells. 
During fertilization these different assortments are brought to- 
gether in random recombinations. We have said that in an 
animal with 8 chromosomes we might have 2 4 or 16 different 
classes of gametes. In random fertilization this number would be 
squared, so that there would be 4 4 or 256 possible combinations. 
Many of these would be duplicates, so that the exact number of 
different classes of zygotes according to their assortment of 
chromosomes would be 3 4 or 81. 

FIG. 31. Chromosomes of Protenor. A, A', male diploid group. B, B', female 
diploid group. The X-chromosomes are indicated by X. (After Wilson.) 

Sex chromosomes, X-O type. In many animals, such as the 
insect Protenor, the male has one chromosome less than the 
female, the numbers in Protenor being 13 and 14, respectively 
(Fig. 31). If the synaptic pairs are assembled, it is clear that the 
male has six pairs of ordinary chromosomes (autosomes) and an 
extra one, the X-chromosome. The female has six pairs of auto- 
somes and a pair of X-chromosomes, In the female the X- 



chromosomes unite in synapsis, form a tetrad, and are segregated 
in the meiotic divisions so that every egg has a complete set of 
autosomes and one X-chromosome (A + X). In the male, on 
the other hand, the single X-chromosome has no synaptic mate 
and so goes on the spindle of the first meiotic division as a dyad, 
which is carried to one pole of the spindle entire. In the second 
meiotic division the dyad is divided as usual. The end result is 
that only half the spermatids receive an X-chromosome, and two 
classes of sperms are formed, either with or without an X-chromo- 
some (A + X or A + 0). If a sperm with an X-chromosome 
fertilizes the egg, the female combination (2 A + 2X) is restored. 

Meiotic Sperm 

FIG. 32. Diagram showing history of the X-chromosome during meiosis and 
fertilization. (After Wilson.) 

If a sperm without the X-chromosome penetrates the egg, the 
male combination (2 A + X] is formed (Fig. 32). 

Sex chromosomes, X-Y type. But the sexes do not always 
differ in chromosome number, for in many animals, like the insect 
Lygaeus (Fig. 33), the X-chromosome of the male is furnished 
with a synaptic mate which differs from it in size, form, and 
probably composition, and is therefore known as the Y-chromo- 

X ' Y 

some. The male forms a tetrad v * v , and the sperms therefore 

J\. i i 

have either an X-chromosome or a Y-chromosome. Fertilization 
by a sperm bearing the X-chromosome results in the develop- 
ment of a female (2 A + 2X), whereas if a sperm bearing a 


Y-chromosome enters the egg the embryo will give rise to a male 
(2A + XY). 

Sex chromosomes, W-Z type. As an exception to the general 
rule among the vertebrates, the birds have dissimilar sex chromo- 
somes in the female. The cytological details are difficult to inter- 
pret but the theoretical explanation is that the female has two 
dissimilar sex chromosomes known as W and Z, while the male 




FIG. 33. Chromosomes of Lygaeus. A, A', male diploid group. B, B', female 
diploid group. X and Y indicate the X- and Y-chromosomes respectively. (After 

possesses two similar sex chromosomes of the Z type (Fig. 34B). 

W :Z 

In the meiosis of the oocyte, therefore, a tetrad Tfrrv is formed, 

and the ovum receives either a W-chromosome or a Z-chromo- 

Z ' Z 
some. The spermatocyte forms a tetrad ^~7~^y and all sperms 

carry one Z-chromosome. In this group, therefore, it is the ovum 
which determines the sex of the embryo rather than the sperm. 
This explanation agrees with the data obtained from genetics. 

CHROMOSOMES OF THE AMPHioxus. The diploid number 
is 24. 

CHROMOSOMES OF THE FROG. The diploid number is 26, 
and the sex chromosomes of the male are of the X-Y type 
(Fig. 34A). 


CHROMOSOMES OF THE CHICK. The diploid number is 35 or 
36. The sex chromosomes have not been positively identified, 
but genetic evidence indicates that the sex chromosomes of the 
female are of the 0-Z or "the W-Z type (Fig. 34B). 

CHROMOSOMES OF MAN. The diploid number, according to 
the most recent researches, is 48. The sex chromosomes are 
of the X-Y type (Fig. 34C). It is interesting to note that 
with 48 chromosomes the possible types of gametes number 2 24 

Fia. 34. Metaphase plates of male diploid chromosome groups. A, frog (after 
Witschi). B, chick (after Hance). C, man (after Painter). 

or 16,777,300, and that from these 3 24 zygote-recombinations are 


It has already been said that the behavior of the chromosomes 
itself might suggest that these bodies are concerned with the 
transmission of hereditary characters. We shall now turn our 
attention to the laws of heredity as worked out by plant and 
animal breeders and learn how the data of genetics agree with 
the data of cytology. 

The unit of genetics is the gene. These genes are arranged in 
linear order in the chromosomes, presumably bound together by 
the chromonemata, and possibly identified with the chromomeres. 
They exist in great numbers; in the fruitfly Drosophila it is 
estimated that there are between 2000 and 3000. Ordinarily 
ultramicroscopic, it has been reported recently by Belling (1930) 
and by Bridges (1934) that they have been able to identify these 
units in material of exceptionally favorable nature. The genes 
are known by the effects their presence induces, and named ac- 
cording to the most obvious of these effects. Thus the Drosophila 


has a gene for (or a gene which induces among other effects) the 
normal type of wing. But there have arisen, among the millions 
of fruitflies raised by geneticists, some with abnormal types of 
wings, such as a vestigial wing. In thiscase there is said to be a 
gene for (or a gene which induces among other effects) the vestigial 
type of wing. 

Dominance. Among the original discoveries of Mendel was 
the fact that, if two organisms with alternative characters were 
mated, the offspring would show either one or the other of the 
characters concerned. This is known as the law of dominance. 
When a Drosophila with normal wings is mated to one with 
vestigial wings, all the offspring have the normal type of wing 
(Fig. 35). Therefore the gene for normal is said to be dominant 
to the gene for vestigial, which, conversely, is said to be recessive 
to the gene for normal. 

It is customary among geneticists to use the initial letter of the 
name for the abnormal character as a symbol for the gene inducing 
its appearance, as well as a symbol for the gene inducing the 
alternative (allelomorphic) normal character. The two are dis- 
tinguished by using a capital letter for the dominant gene, a 
lower-case letter for the recessive gene. In this case, then, the 
symbol of the gene for vestigial is v, and the symbol of the gene 
for normal is V. 

Every adult has two haploid sets of chromosomes, and therefore 
a pair of every kind of chromosome. If both members of a pair 
have the same gene (w or FF) they are said to be homozygous; 
but if one chromosome has the dominant, and the other has the 
recessive gene (Vv\ they are said to be heterozygous. 

The individuals that are mated together in the first instance 
are known as the parental generation (Pi); their offspring are 
known as the first filial generation (Fi) ; the next generation is 
the second filial generation (F 2 ) ; and so on. 

Segregation. In the experiment where a normal long-winged 
fly was mated with a vestigial- winged fly, the long- winged parent 
must have had two chromosomes each containing the dominant 
gene F, for all the offspring (Fi) showed this dominant character. 
The vestigial-winged parent must have had two chromosomes 
containing the recessive gene v. In the maturation of the gametes 
all the sperms received F, while the eggs all received v, 



FIG. 35. Diagram to show the effects of crossing two flies differing in respect to one 
pair of genes. V is used for the dominant gene for the normal character long 
wings; v is used for the recessive mutant gene for vestigial wing. (From Curtis 
and Guthrie, after Morgan ei al.) 

The Fi flies have the genetic constitution Vv, that is to say, 
they are heterozygous. When they are mated to each other the 
eggs will receive a chromosome containing the gene V or the gene 
v, and the same is true of the sperm. 



The F 2 flies will consist of three genetic groups (genotypes) 
because of random fertilization, namely, homozygous long- winged 
flies (VV), heterozygous long- winged flies (F#), and vestigial- 
winged flies (w). From Fig. 36, it will be seen that the ratio 
will be one homozygous long-winged fly to two heterozygous 
long-winged flies, and to one vestigial-winged fly. Or one may 
say that there are two recognizable classes of adults (phenotypes), 
in the ratio of three long-winged flies to one vestigial-winged fly. 
This is the famous Mendelian ratio applied to the inheritance of 



FIG. 36. Diagram to show the segregation of the genes caused by the distribution 
of the chromosomes to the gametes and zygotes of the FI and F 2 generations. 
F, and v as before. 

one pair of allelomorphic characters, or ,as we should say today, 
to one pair of genes. 

Evidently Mendel's law of segregation may be stated in terms 
of the gene theory as follows: allelomorphic genes are segregated 
during maturation into different gametes. 

Assortment. It is an amazing coincidence that Mendel stud- 
ied the inheritance of seven pairs of allelomorphic characters in 


the edible pea, a species which has seven pairs of chromosomes, 
and that the genes for each pair of characters were located in a 
different pair of chromosomes. 

When a Drosophila with vestigial wings and normal gray body 
color is mated to a fly with normal long wings and ebony body 
color, the Fi flies are gray-bodied and long- winged. Evidently 
the gene for gray body (E) is dominant to the gene for ebony 
body (e). That the genes for these characters are independent 
of those affecting wing length is shown when the hybrid FI flies 
are mated together. Four classes of phenotypes result in the F 2 
generation: 9 long- winged, gray-bodied; 3 long- winged, ebony- 
bodied; 3 vestigial- winged, gray-bodied; and 1 vestigial- winged, 
ebony-bodied. This ratio of 9 : 3 : 3 : 1 breaks down to 3 long 
to 1 vestigial, and 3 gray to 1 ebony, demonstrating mathe- 
matically that two pairs of factors are involved. 

It is evident that the problem involves the segregation of two 
pairs of chromosomes (Fig. 37) . The genetic constitutions of the Pi 
flies were wEE and Wee, respectively. The gametes receive one 
chromosome from each pair of synaptic mates, so the genetic con- 
stitution of the eggs is vE and that of the sperms Ve (or vice versa). 

The Fi flies have the formula VvEe, and their gametes, because 
the chromosome pairs are assorted independently, will belong to 
four classes : VE, Ve, vE, and ve. 

The F 2 flies as seen from the checkerboard diagram will fall into 
16 combinations, which by canceling the duplicates reduce to 9 
genotypes (VVEE, VVEe, VvEE, VvEe, Wee, Vvee, wEE, 
vvEe, and wee), and 4 phenotypes as listed above. 

Mendel's law of assortment may be phrased in terms of gene 
theory as follows: different pairs of allelomorphic genes when 
located in different pairs of chromosomes are assorted independently 
during maturation into different gametes. 

It may be noted that if n stands for the number of pairs of 
genes located in different pairs of chromosomes, then 2 n represents 
the number of gamete classes formed by the Fi generation; 2* 
represents the' number of phenotypes in the F 2 generation; 3* the 
number of genotypes in the F 2 generation; and 4 n the number of 
combinations in the Punnett square. The number of individuals 
in each phenotype is obtained by expanding the 3 : 1 formula as 
follows: (3 : 1), (9 : 3 : 3 : 1), (27 : 9 : 9 : 9 : 3 : 3 : 3 : 1) . , . . 





Zygotes from 
which P l 

Gametes of P l 

Zygotes of F t 
that give rise 
to long-winged, 
gray-bodied flies 

Zygotes of F a 

9 long-winged, 

3 long-winged, 

3 vestigial- winged, 

1 vestigial-winged, 

FIQ. 37. Diagram to show the assortment of two pairs of genes due to the dis- 
tribution of two pairs of chromosomes. E, gene for gray body; e for ebony body; 
F, and v as before. (From Curtis and Guthrie.) 


Linkage. The characters with which Mendel worked segre- 
gated freely, showing that their genes were not borne in the same 
chromosome. Later studies have shown that some characters do 
not segregate, and this leads to the assumption that their genes 
are carried in the same chromosome and therefore are inherited 

When a Drosophila with gray body color and long wings is 
mated to one with black body color and vestigial wings, the Fi 
flies are gray-bodied and long-winged. Note that the gene for 
black (b) will act very differently from the gene for the similar 
color ebony (e). If the FI flies are bred together a very confusing 
ratio appears in the F 2 generation: practically all the flies are 
gray-bodied and long-winged or black-bodied and vestigial-winged 
like the Pi generation, but there are only a few individuals repre- 
senting the other classes we might expect under Mendel's law of 
assortment. If we make a reciprocal cross between a long-winged 
black-bodied fly and a vestigial-winged gray-bodied fly, the F 2 
flies are all of these two (Pi) types with few exceptions. This 
continued association of two genes through several generations is 
called linkage and suggests that the associated genes are located 
in the same chromosome. 

This theory may be tested by back-crossing (Fig. 38) a male of 
the FI generation (BbVv) to a double recessive female (bbvv). 
All her eggs will have the recessive genes (bv). We can then test 
the constitution of the sperm by examining the progeny of this 
cross (here called F 2 for convenience), for all the F 2 flies must have 
the genes (bv) from the mother. The flies of this generation are 
either gray-bodied and long-winged (BbVv) or black-bodied and 
vestigial- winged (bbvv). This seems to show that the genes B and 
V were located in one chromosome while 6 and v were located in 
the synaptic mate. 

Crossing over. Now for the exceptional (cross-over) flies 
noted above. There is no crossing over in the maturation of 
the male FI fly, but how about the female? When we mate 
(Fig. 39) a female FI fly (BbVv) to a double recessive male (bbvv), 
the progeny (F 2 ) fall into four classes: 41 per cent gray-bodied 
and long-winged (BbVv), 41 f per cent black-bodied and vestigial- 
winged (bbvv), 8$ per cent gray-bodied and vestigial-winged 
(Bbw), and 8 per cent black-bodied and long-winged (66 Vv). 



Obviously there has been an exchange of some sort between the 
chromosomes of the female Fi fly. Both cytological and experi- 

FIG. 38. Diagram to show the inheritance of two pairs of genes when located in 
one pair of chromosomes, (linkage). In this case the male FI fly is back-crossed 
to a double recessive female. B, gene for gray body; 6, for black body; V, and v 
as before. (After Morgan.) 

mental evidence seem to indicate that this crossing over takes 
place in the prophase of the first meiotic division (Fig. 29, 4)- 
Although there are still difficulties in determining exactly how 
the crossing over takes place between the four strands, it is gener- 



ally agreed that the actual crossing over takes place between two 
of them. The idea of linkage between genes in the same chromo- 

41.5 8.5 8.5 41.5 

FIG. 39. Diagram to show the inheritance of two pairs of genes when located in 
one pair of chromosomes between which crossing over takes place. In this case 
the female FI fly is back-crossed to a double recessive male. Symbols as in Fig. 
38. The figures at the bottom of the illustration indicate the percentage of each 
phenotype in the entire hatch. (After Morgan.) 

some suggested the idea that the genes form a longitudinal series 
in each chromosome. This is supported by the behavior of the 



chromosomes in ordinary somatic mitosis, in synapsis, and in 
crossing over (Fig. 40). 

Finally Sturtevant (1913) suggested that the percentage of 
crossing over between two pairs of linked genes might represent 
a function of the distance between the loci of the genes in 
the chromosome. Accordingly, maps have been constructed, by 
Morgan and his co-workers, on the general assumption that one 
per cent of cross-overs is represented on the map by a distance of 
one unit between the genes involved. Without going into further 
details of the methods used in constructing these maps, for there 

FIG. 40. Diagram to illustrate: A, splitting of a chromosome in somatic mitosis; 
B, union of two chromosomes in synapsis; C, union of two chromosomes in synap- 
sis accompanied by crossing over. (After Wilson.) 

are many complicating factors, a glance at the accompanying 
chart (Fig. 41) will show the progress that has been made in this 

Sex-linked inheritance. One of the most striking evidences 
that genes are borne in the chromosomes is afforded by what is 
known as sex-linked (criss-cross) inheritance. This is illustrated 
in Drosophila by the inheritance of white eye color, an allelomorph 
of red, the normal eye color. If a white-eyed male is mated with 
a red-eyed female (Fig. 42), the Fi flies of both sexes will have red 
eyes. But if these F\ flies are bred together the F 2 generation 
will be made up of red-eyed females, (50 per cent) red-eyed males 
(25 per cent), and white-eyed males (25 per cent). It looks at 
first like an ordinary 3 to 1 Mendelian ratio, except for this 
curious distribution of eye color in the two sexes. 


~ (0. yellow CB) 
>/ XO Hairy wing CW) 

T 0. telegraph (W) 
| 2. Star CE) 

: \ \Q6 broad CW) 
\ 1. prune CE) 
\ \ 1.5 white CE) 

( 6. expanded Cw) 

' \I3. facet CE) J 

I 12.t Gull CW) 

\ ]3. Notch CE) 

[ 13. TruncateCw) 

\ 4.5 Abnormal CB) -i 

14. dachsous CB) 

\ \55 echinus CE) J 

16. Streak CB) 

\\6.9 bifid CW) 

\\75 ruby CE) 

\ 137 crossveinless CW) 1 

0. roughoid CE) 


20. cut CW) 

21. singed CH) 

27.5 tan CB) 
27.7 lozenge CE) 

L 31. dachs CB) 
35. Ski- IE CW) 

26. sepia CE) 
26.5 hairy CB) 

33. vermillionCE) 1 1 

36.1 miniature CW) +41. Jammed Cw) J- 35. rose CE) 

= 36.2 dusky (W) 1 1 36.2 cream-DI CE) 

38. furrowed CE) 1 46 M inute-e CH) 1 40.1 Minute -h CH) 

J.4S- 5 blackCB) * 40.2 tilt Cw) 

43. sable (B) 
- 44.4 garnet GO 

"48.7 jaunty (w) 

" 40.4 Dichaete CH) 
42.2 thread CB) 

54.5 purple CE) 

44. scarlet CE) 

57.5 cinnabar CE) 

48. pmk CE) 

64.2 small wing 
' 54.5 rudimentary (VV)' 
56.6 forked CH) 

60. safraninCE) : 

49.7 maroon CE) 
\J50.t dwarf CB) 
|50. curled CW) 

: 57. Bar CE) 
585 small eye 
: 59. fused (>V) 
. 59.6 BeadexCW) 
62. Minute-n (H) ' 

64.1 pink-wing CEW) 

67. vestigial Cw) i 
68. tel escape CW) * 

54.8 Hairy wing supr 
58.2 Stubble 00 
58.5 spinelessCH) 
58.7 bithoraxCB) 
59.5 bithorax-b 

65. cleft CW) 

72. Lobe CE) 

62. stripeCB) 

10. bobbedCH) : 

74. qap (W) 
75.5 curved Cw) 

- 69.5 hairless CH) 


- 70.7 ebony CB) 
72. bcmdCB) 


'75.7 cardinal CE) 

83.5 frfngedCw) 

' 762 white ocelli (fc) 


h90. humpy CB) 

91.1 rough CE) 

^''(f' : 

99.5 arcCW) 
100.5 plexus CW) 
. 102. Tethal-Ha 

93. crumpledCW) 
: 93.8 Beaded CW) 
94.1 Painted CW) 

n m 

(105. brown CE) 
'1105.1 blisteredO^) 
. 106. purpleoid CE) 
J07.t moruJa (J) 

100.7 claretCE) 
1 101. Minute CH) 

"* 107.5 balloon (>V) 

!06.2 Minute-g (H) 


eyeless CE) 
rotated CB) 

| male fertility 

male fertility 1 

FIG. 41. The chromosomes of Drosophila melanogaster and map showing the 
positions of many genes as determined from cross over ratios. Letters in paren- 
theses indicate part of body affected: B, body; E, eye; H,hair; W, wing. ' Arrows 
indicate position of attachment point. All genes in IV are closely linked. The 
exact position of genes in Y still undetermined. (From Sharp after Morgan et al. 
1926 and Stern 1929.) (8 2) 



In the reciprocal mating, a red-eyed male to a white-eyed 
female (Fig. 43), the Fi generation is made up of red-eyed females 

FIG. 42. Diagram to show the inheritance of one pair of genes when located in the 
X-chromosome (sex-linkage). W, gene for red eye (dominant, normal); w, gene 
for white eye (recessive, mutant). The empty hook-shaped chromosome repre- 
sents the Y-chromosome. N. B. In the text the X-chromosome, whjn bearing 
w, the gene for white eyes, is designated by a small x. In this cross, red-eyed 
female is mated to a white-eyed male. (After Morgan et al.) 

and white-eyed males (criss-cross inheritance). When these F\ 
flies are bred together, there are four classes of flies in the F 2 
generation : red-eyed males and females, and white-eyed males 



and females (25 per cent in each class). This is not a Mendelian 
ratio, but it can be explained on the assumption that the gene for 

JIG. 43. The reciprocal cross to that shown in Fig. 42. A white-eyed female is 
mated to a red-eyed male. Symbols as in Fig. 42. (After Morgan et al.) 

white eye color (and its allelomorphs, of which there are several) 
is located in the X-chromosome. 

Let us use the symbol X for an X-chromosome bearing a 
gene for red, x for an X-chromosome bearing a gene for white, 
and Y for the Y-chromosome. In the first genetic experiment, 
formulas for the parental generation are XX (red female) and 
x Y (white male). All the eggs receive an X, the sperms either 



x or F. Consequently the Fi generation is made up of flies with 
the formula Xx (heterozygous red female) and XY (red male). 
The eggs of this generation receive X or x y the sperms X or F. 

Two pairs of homologous chromosomes 
showing positions of allelomorphic genes. 

Crossing over: The chromosomes of the 
pair shown in A may twist about one another 
as in C and break in the plane of the dotted 
line so that comparable sections are ex- 
changed as shown in D. 

Deletion: One member of the chromosome 
pair shown in A may twist on itself as in E 
and break in the plane of the dotted line so 
that an internal section containing gene c is 
lost, or deleted, as shown in F. 

Inversion: One member of the chromosome 
pair shown in A may twist on itself as in G 
and break in the plane of the dotted line so 
that the section containing genes B and C is 
inverted as shown in H. 

Duplication and Deficiency: If one member 
of the chromosome pair shown in A comes to 
lie across the other as shown in I arid a break 
occurs in the plane of the dotted line, the 
chromosome on the left in J will have a 
duplication and contain both gene d and 
gen D, while the chromosome on the right 
will have a deficiency of the section contain- 
ing gene D. 

Translocation: One member of the chro- 
mosome pair shown in A may come to lie 
across one member of the chromosome pair 
shown in B, as seen in K. If a break occurs 
in the plane of the dotted line, section's of 
non-homologous chromosomes are exchanged, 
or translocated, as shown in L. ^ *-* 

FIG. 44. Diagrams to show crossing over and various chromosomal aberrations. 
(From Curtis and Guthrie.) 


1. i 











A B 

a (iM a 


^V 6 * 


c {"Stf c 


d{ {> d 


C D 


A a 



B b 




c d 





E F 


A a 







D c 





G H 





J^i 6 






7 (f 



I J 

a A\ a\ 



1 I 

b B\ fl 



\ / 


2 II 

C C\i/2 



d 8 /N j 




So the F 2 generation is composed of flies with the following com- 
binations: XX (homozygous red females), Xx (heterozygous red 
females), XY (red males), andxY (white males). 

In the other experiment the parental formulas are xx (white 
female) and XY (red male). The eggs receive an x, the sperm X 
or Y. Hence there are two classes in the FI generation, xX 
(heterozygous red females) and xY (white males). The eggs re- 
ceive either an x or an X, the sperms receive either x or Y. The 
four combinations possible in the F 2 generation are xX (hetero- 
zygous red female), xx (white female), xY (white male), and XY 
(red male). 

Chromosomal aberrations. Crossing over takes place between 
the two X-chromosomes, but apparently not between the X-chro- 




Normal Female 


Normal Male 

(both X*a from Mother) 


FIG. 45. Diagrams showing I, normal disjunction of X-chromosomes in oogenesis, 
and fertilization by two types of sperms; II, non-disjunction, both X-chromosomes 
remaining in egg; III, non-disjunction, both X-chromosomes passing to polocyte. 
A y one haploid set of autosomes. (From Curtis and Guthrie.) 

mosome and the Y-chromosome in Drosophila. We have already 
noted the fact that in this little fruitfly crossing over does not 
take place in the male. 1 But crossing over by no means exhausts 
the possibility of effecting new combinations of genes by the be- 
havior of the chromosomes during the maturation of the germ 
cells. Exact as the mechanism of meiosis may seem, many possi- 

1 There is some recent evidence to show that such crossing over can be induced 
by high temperatures. 



bilities of disturbance have been discovered by genetic and cyto- 
logical methods. 

The accompanying diagram (Fig. 44) illustrates graphically 
some of the aberrations which may take place during meiosis. 
These result in the appearance of unexpected individuals with 
new combinations of genes 

FIG. 46. Intersexes and supersexes in Drosophila, occurring in the progeny triploid 
females. A, female-type intersex. B, male-type intersex. C, superfemale. 
D, supermale. a, b, and c are the chromosome groups characteristic of A, B, 
and C respectively. (From Curtis and Guthrie, after Morgan et al.) 

Non-disjunction. A special type of chromosomal aberration 
is one in which the two members of the synaptic pair may fail to 
separate during the meiotic divisions, so that one egg receives, for 
example, two X-chromosomes (A + 2X), while another receives 
none (A) (Fig. 45). When fertilized by a sperm with an X- 
chromosome, the egg with two X-chromosomes, if it develops 
into an adult, will be a superfemale (2 A + 3 X) differing markedly 
from her sisters (Fig. 46C). When fertilized by a sperm with a 


Y-chromosome, the egg without any X-chromosomes (2A + Y) 
dies. The other possible combinations are shown in the diagram. 
In some cases all the chromosomes fail to disjoin so that an egg 
receives a diploid set of chromosomes (2 A + 2 X). When ferti- 
lized by an A + X sperm it becomes a triploid female (3 A + 3X). 
The eggs formed by these triploid females may have the formula 
2 A + X or A + 2X. If an egg of the first type (2 A + X) is 
fertilized by sperm carrying an X-chromosome (A + X], the 
zygote will have the formula 3 A + 2X. Such a zygote develops 
into an abnormal fly known as an intersex (Fig. 46 A), male in 
some respects and female in others. Superfemales (2 A + 3X) 
may also arise from the egg of the second type (A + 2X) being 
fertilized by an A + X sperm. Supermales (3 A + -XT), on the 
other hand, arise from the fertilization of a 2A + X egg by an 
A + Y sperm (Fig. 46D). It would appear from these formulas 
as though the determination of sex depended on some sort of ratio 
between the genes in the X-chromosomes and the autosomes, and 

Bridges (1921) has formulated a theory of 
genie balance to account for the observed 

Gynandromorphs. Intersexes must not 
be confused with gynandromorphs, which 
are individuals with one part of the body 
male and the rest female. Bilateral gy- 
nandromorphs in Drosophila (Fig. 47) arise 
from female zygotes (2 A + 2X), but during 
the first cleavage division one of the X- 
FIG. 47 -Gynandromorph c h rO mosomes is lost on the mitotic spindle. 

in Drosophtla melanogas- . in 

ter. Note eosin eye and The result is that one of the daughter cells 
miniature wing on right has the female complex (2 A + 2X) while 

as compared to red eye the other hag the male comp lex (2 A + X). 
and long wing on left. ~ . , , . x i i 

This fly is male on the Sometimes such an aberration takes place 
right side and female on in a later cleavage division so that there 

the left. (After Morgan | s on l y a gma Jl area o f ma l e ce U s> 

and Bridges.) Teratology. All students of embryol- 

ogy are familiar with the fact that development does not always 
proceed normally. Abnormal embryos are known as monsters, 
and their study forms the subject matter of the embryological 
subscience known as teratology. It is clear from the sections 


just preceding that many of these monsters must be due to 
chromosomal aberrations with consequent disturbance of the 
genie balance. Others, as will be noted in Chapter VII, are due 
to environmental factors. 

Mutations. So far we have considered the genes as though 
they were immutable. But the question naturally arises as to 
the origin of the genes which are allelomorphic to the so-called 
normal genes. In Drosophila the abnormal genes, or mutants as 
they are called, arose in laboratory cultures. It has been dis- 
covered that the rate of mutation, i.e., the number of mutants 
arising in a given number of flies, may be increased by high tem- 
peratures (Plough) and by irradiation (Miiller). When one of 
these genes is altered in any way to become a mutant, the course 
of development is disturbed. Most mutant genes disturb the 
course of development so greatly as to cause death (lethal 
mutants). A smaller number produce visible changes when 
present in each chromosome of the synaptic mates (recessive 
mutants). A few produce visible changes if contained in a single 
chromosome (dominant mutants). Accordingly, every species 
of animals contains a certain number of mutant genes (400 in 
Drosophila). As these enter into new genetic combinations ac- 
cording to the behavior of the chromosomes in meiosis and fertil- 
ization, they give rise to individual differences in development. 
But the greater number of stable or non-mutant genes holds 
development true to the specific type. 

One of the outstanding problems in experimental embryology 
still awaiting solution is the question how the genes actually 
determine the course of development. But the modern student 
of embryology accepts the general theory that it is the comple- 
ment of genes, from the egg and sperm respectively, which 
initiates and largely controls the development of the individual. 


The egg and sperm are the material contributions of the parents 
to the new individual. The equivalent structures of the egg and 
the sperm are their nuclei. Each nucleus contains the haploid 
number of chromosomes. The fertilized egg has two haploid 
sets, or the diploid number. In somatic mitosis the chromosomes 
are split longitudinally and divided equally among the daughter 


cells, so that each daughter cell contains an assortment precisely 
equivalent to that of its sister cell and the mother cell. In the 
course of the meiotic divisions the diploid number of chromosomes 
is reduced to one haploid set. This is accomplished through the 
union of the homologous members of the two sets in synapsis. 
Each synaptic pair forms a tetrad of four chromatids, the members 
of which are distributed independently among the mature germ 
cells. In this way different classes of gametes are formed with 
varying chromosomal complexes. 

The chromosome is built up from a thin thread, the chro- 
monema, which binds together the genes, the units of heredity, 
provisionally located at nodes of the chromonema called chromo- 
meres. These genes, ordinarily ultra-microscopic, are self-repro- 
ducing units which seem to accelerate definite chemical reactions 
without losing any of their own substance in the process. The 
course of development is largely controlled by the activities of 
these genes. These activities may be disturbed during meiosis 
by chromosomal aberrations, thus altering the genie balance and 
modifying the course of development, in some cases so much as to 
cause death. The genie balance may also be altered by point 
mutations or changes in the constitution of an individual gene 
recognizable through the effects produced. 

Either aberrations or point mutations when not lethal may be 
transmitted in heredity. The distribution of these aberrant 
chromosomes or mutant genes in meiosis and fertilization is the 
material basis for heritable differences arising in the course of 
development of individuals belonging to the same species. 


Cowdry, E. V. (ed.) 1924. General Cytology, Sections X, XI. 

Morgan, T. H. 1913. Heredity and Sex. 

1919. The Physical Basis of Heredity. 

/- 1922. The Mechanism of Mendelian Inheritance, 2nd Ed. 

^ 1934. Embryology and Genetics. 

/- and others. 1928. The Theory of the Gene. 

Sharp, L. W. 1934. Introduction to Cytology, Chaps. 17-24. 

Wilson, E. B. 1925. The Cell, etc., Chaps. &-12. 


The fertilized egg (zygote) is a complete and balanced cell. 
It has two entire sets of chromosomes, each with a full comple- 
'ment of genes, one set from each parent. These nuclear elements 
are contained in a cell body whose ' cytoplasm is principally 
maternal in origin and which has a definite organization as in- 
dicated by its polarity. We are now to examine the way in 
which the embryo develops from the fertilized egg. 

It is customary to distinguish three steps in the early develop- 
ment of the embryo. First is the period of cleavage in which the 
egg undergoes a number of mitotic cell divisions at each of which 
the number of cells (blastomeres) increases while the size of the 
cells decreases. The period ends with the embryo in the form of 
a blastula, a sphere or disc in which the blastomeres are not 
stratified into different layers. 

Second comes the period of gastrulation in which the blasto- 
meres arrange themselves into an outer and inner layer of cells, 
known as ectoderm and endoderm, respectively. This two- 
layered embryo is called a gastrula. 

Third is the period in which a middle layer, including the meso- 
derm and the notochord, is formed between the ectoderm and 
endoderm. Although this layer sometimes develops during gas- 
trulation, it is customary to distinguish a period of mesoderm 
(chorda-mesoderm) formation. This distinction is not always 
valid, nor is it important, for, as will be seen, the material which 
is to form the middle germ layer can sometimes be distinguished 
in gastrulation, cleavage, or even in the fertilized egg. 


As there are different types of eggs according to the amount 
and distribution of the yolk, so there are different types of 
cleavage according to the pattern formed by the dividing egg. 

Rules of cleavage. Certain rules have been formulated to 
express the simpler geometrical relationships of the blastomeres. 




The first are those of Sachs: (1) cells typically tend to divide into 
equal parts; (2) each new plane of division tends to intersect the 
preceding one at right angles. Sachs's rules are supplemented, 
and to some extent explained, by those of Hertwig: (1) the 
typical position of the nucleus (and hence of the mitotic figure) 
tends towards the center of its sphere of influence, i.e., of the pro- 
toplasmic mass in which it lies; (2) the axis of the spindle typi- 
cally lies in the longest axis of the protoplasmic mass, and division 
therefore tends to cut this axis transversely. 

Methods of cleavage. The rate of division is governed by 
the rule of Balf our : the rate of cleavage is inversely proportional 
to the amount of yolk present. This leads to a distinction be- 
tween two types of cleavage. In the first type the cleavage 



FIG. 48. Diagram to show main types of cleavage in vertebrates. A, equal holo- 
blastic. B, unequal holoblastic. C. meroblastic. 

planes divide the egg completely into separate blastomeres. 
This is known as holoblastic cleavage, and is characteristic of 
isolecithal and moderately telolecithal eggs. In the second type 
the cleavage planes do not pass through the yolk and so the 
separate blastomeres come to lie upon a mass of undivided yolk. 
This is known as meroblastic cleavage and is typical of extremely 
telolecithal eggs. It is generally true that isolecithal eggs have 
equal holoblastic cleavage (Fig. 48A). Moderately telolecithal 
eggs have unequal holoblastic cleavage (Fig. 48B), and extremely 
telolecithal eggs have meroblastic cleavage (Fig. 48C). 

Cell lineage. It must not be thought that cleavage results in 
a mass of identical blastomeres. Painstaking examination of 
dividing eggs has shown that in the normal development of 
favorable material the origin and fate of every blastomere can 
be determined accurately. The genealogical history of the blasto- 



meres is known appropriately as cell lineage. One of the most 
clean-cut examples, in forms allied to the vertebrates, is the cell 
lineage of the tunicate Styela (Cynthia), worked out by Conklin 
in 1905. The accompanying diagram (Table 6) shows the cell 
lineage up to the 32-cell stage with the ultimate fate of each of 
the blastomeres. 

In reading this chart the student should understand the system 
used in naming the blastomeres, which is illustrated most easily 

C ^ ^ D 

FIG. 49. Cleavage of Styela (Cynthia) egg. A, 4-cell stage from left side. B, same 
stage from animal pole. C, 8-cell stage from left side. D, same stage from animal 
pole. For explanation of lettering see text. (From Richards, after Conklin.) 

by means of the 8-cell stage (Fig. 49). The blastomeres which 
will give rise to structures on the right side of the embryo are 
underlined. The blastomeres formed at the animal hemisphere 
are in lower-case letters; those at the vegetal hemisphere are in 
capital letters. Those formed at the antero-dorsal side of the 
embryo are given the designation A or a; those at the postero- 
yentral side are named B or b. The first exponent is the number 
of the cell generation, counting the fertilized egg as the first 



generation, the blastomeres of the first cleavage as the second 
generation, etc. The exponent after the decimal point indicates 
whether the cell is in the first, second, third, etc., row from the 
vegetal pole. Thus the cell labelled A 4<1 is antero-dorsal, left 
side, vegetal hemisphere, of the fourth generation, and in the row 
next to the vegetal pole. 

CELL LINEAGE OF Styela (Cynthia) AFTER CONKLIN (1905) 

Number of cells 
Ectoderm (epidermis) 









[ a 5 - 4 

f a 6 - 8 
1 a 6 - 7 


AB 2 


A 3 

A 4.1 

b 4 - 2 

B 4.1 

A 5 - 2 

A 5 - 1 

b 6 - 4 

b 8 - 3 


AB 2 


A 3 

B 3 

A 4 - 1 

b 4 - 2 

B 41 

a 5 - 3 
A 5 - 2 


b 6 - 4 
b 5 - 3 

I 5 ' 2 
B s.i 

A 6 - 4 

" (neural plate) 
Chorda-neural plate 
Chorda-neural plate 

Mesoderm (gray crescent) 

U H ft 

(yellow " ) 
Ectoderm (epidermis) 

" (neural plate) 
Chorda-neural plate 
Chorda-neural plate 

Mesoderm (gray crescent) 



The first cleavage is bilateral; i.e., it divides the egg, with its 
presumptive organ regions, into a right blastomere (AB 2 ) and a 
left blastomere (AB 2 ). At the second cleavage each of these is 


divided into an antero-dorsal blastomere (A 3 and A 3 ) and a 
poster o- ventral blastomere (B 3 and B 3 ). The third cleavage 
plane (Fig. 49C, D) separates the smaller cells of the animal hemi- 
sphere (a 4 - 2 , b 4 2 , a 4 - 2 , b 4 - 2 ) from the larger cells of the vegetal hemi- 
sphere (A 4 - 1 , B 4 - 1 , A 4 - 1 , B 4 - 1 ). 

By the sixth generation (32-cell stage) the organ-forming re- 
gions have been segregated into different blastomeres as follows : 

Animal hemisphere : 

14 Ectoderm, epidermis. 
2 Ectoderm, neural plate. 

Vegetal hemisphere : 

4 Ectoderm and mesoderm, chorda-neural plate. 
4 Mesoderm, gray crescent. 
2 Mesoderm, yellow crescent. 
6 Endoderm cells. 

The cell lineage of many types of invertebrates has been in- 
vestigated in a similar manner, and as a result it is now generally 
recognized that during cleavage the successive generations of 
blastomeres show a progressive differentiation. Earlier or later, 
the presumptive organ regions of the fertilized egg are segregated 
into different groups of blastomeres, each group forming a pre- 
sumptive organ region of the blastula (page 102). 

Later (Chapter VII), experiments will be described which indi- 
cate that individual blastomeres may, under different conditions, 
give rise to parts of the embryo other than those which they 
produce in the normal course of development. 

CLEAVAGE-. THE AMPHIOXUS. In the egg of the amphioxus 
(Fig. 50), which is isolecithal, cleavage is holoblastic and almost 
equal. The first cleavage commences as a depression at the 
animal pole, which later assumes a groove-like form and elongates 
until it becomes a wide meridional furrow extending around the 
egg. This constriction deepens until the two hemispheres are 
completely divided, when each blastomere rounds up into a 
spherical shape. The second cleavage also commences at the 
animal pole and is meridional but at right angles to the first, 
following the second rule of Sachs. The third plane of cleavage 



is at right angles to both the first and second and hence would be 
equatorial if the egg were completely isolecithal. But as the yolk 
is a little concentrated at the vegetal pole, the nucleus, following 
Hertwig's first rule, is in the center of the protoplasm, i.e., on the 
egg axis slightly nearer the animal pole. So the third cleavage 
plane is nearer the animal pole and accordingly is latitudinal. 
The quartette of cells in the animal hemisphere is therefore smaller 
than those in the vegetal hemisphere. The smaller cells are called 
micromeres; the larger ones, macromeres. The fourth division 



FIG. 50. Cleavage of the amphioxus egg. A, before cleavage. B, commencing 
first cleavage, from posterior side. C, second cleavage, from vegetal pole. D, 
third cleavage, from left side. E, fourth cleavage, from vegetal pole. F, fifth 
cleavage, side view, segmentation cavity indicated by dotted lines. X166. 
(After Conklin, 1932.) 

divides each of the eight existing blastomeres in two. There are 
two planes of cleavage, each meridional, at right angles to the 
third, and also at right angles to each other. Sometimes the 
cleavage planes of the fourth division are parallel to each other 
instead of being at right angles. This makes the bilateral 
symmetry of the dividing egg quite obvious. In the fifth cleavage 
32 cells are produced, again by two planes of cleavage, at right 
angles to the planes of the fourth, but this time latitudinal and 
parallel to each other. From this time on cleavage becomes more 
and more irregular. The early cleavages have been fairly regular; 



each has divided the entire egg mass; and the blastomeres, with 
the exceptions noted, have been almost equal. The blastomeres 
round up as each cleavage is completed, and a jelly is secreted 
between them. In this way a small cavity called the segmenta- 
tion cavity or blastocoel is formed. 

Conklin (1933) states that comparison of the cleavage of the 
amphioxus with that of the tunicates shows a general resemblance 
between the two in the distribution of the organ-forming sub- 
stances to the blastomeres, in the generally bilateral type of 
cleavage, and the order of division; but in all respects the tunicate 
egg is the more precise and the more precocious in differentiation. 

CLEAVAGE: THE FROG. The frog's egg (Fig. 51) is telolecithal 
with holoblastic unequal cleavage. Here the first division com- 

D E F 

FIG. 51. Cleavage of the frog's egg. A, third cleavage. B, fourth cleavage (12 
cells). C, fifth cleavage. D, sixth cleavage. E, F, later stages. (After Morgan.) 

mences as a depression at the animal pole, which elongates and 
extends around the egg as a shallow furrow until the ends meet 
at the vegetal pole. The constriction extends inwards and even- 
tually bisects the egg into two blastomeres which round up very 
slightly. The plane of second division is also meridional and 
through the animal pole but at right angles to the first. The first 
two cleavage planes intersect each other at the animal pole; but 
as the blastomeres round up, the planes no longer form a cross, 


but two blastomeres are pushed away from each other, while the 
other two are in contact forming a short polar furrow between 
them. The third cleavage is latitudinal, about 20 above the 
equator, and the micromeres are considerably smaller than the 
macromeres. Theoretically the fourth and fifth planes of cleav- 
age bear the same relationships to the earlier ones as do those of 
Amphioxus, but actually they are more irregular. The two planes 
of the fourth cleavage often fail to pass through the vegetal pole 
and hence become vertical rather than true meridional planes. 
As these planes originate in the animal hemisphere, the micro- 
meres are divided before the macromeres, so that a 12-cell stage 
intervenes between the 8-cell and 16-cell stages. Similarly, fol- 
lowing Balfour's rule, the latitudinal cleavage plane in the animal 
hemisphere of thejifth division appears before the corresponding 
>- plane in the vegetal hemisphere, 

so that there is a 24-cell stage 
before the 32-cell stage is attained. 
The cell lineage of the frog's egg 
has not been followed in detail as 
A ^"TP has that of the tunicate or amphi- 

FIG. 52. The gray crescent of the OXUS. It is known, however, that 

frog's egg in early cleavage. A, first the first cleavage plane ordinarily 
cleavage, posterior view. gggiird divideg the crescent into two 

cleavage, from left side. X}$<abmi- J . , ,_. 

diagrammatic. "W symmetrical halves (Fig. 52A), so 

that cleavage is normally bilater- 
ally symmetrical from the outset. The blastomeres receiving the 
gr^jr crescent material will give rise to notochord and neural 
plate in later development. 

CLEAVAGE: THE CHICK. In telolecithal eggs with meroblastic 
cleavage such as that of the fowl, only the protoplasm of the egg, 
i.e., the blastodisc, is divided, and the cleavage planes do not seg- 
ment the yolk (Fig. 53). The first furrow commences at the 
animal pole and extends outwards towards the edges of the blasto- 
disc. The second is formed by two furrows, at right angles to 
the first, one in each blastomere, which grow towards the first fur- 
row and also towards the edge of the blastodisc. They may join 
the first furrow at approximately the same point or at separate 
points, in which case a polar furrow is formed. These four cells 
are incomplete, as the furrows do not extend all the way to the 



yolk nor to the edge of the blastodisc, but remain connected both 
below and at their margins. From this point on, Cleavage is 
irregular. Some cleavage planes are circular and cut off central 
cells from marginal. These may be compared with the latitudinal 
planes of the holoblastic type. Others are radial, like the first 

C D 

FIG. 53. Cleavage of the hen's egg. A, first cleavage. B, second cleavage. 
C, third cleavage. D, later cleavage. All from animal pole. Approx. X12. 
(A, B, D, after Kolliker; C, after Patterson.) 

and second. Still others are tangential and divide the central 
cells into upper and lower layers, as in the frog's egg. 


human ovum has not yet been observed, but in the egg of the 
monkey (Fig. 54) and rabbit (Fig. 55) the cleavage is clearly of the 
equal holoblastic type. In the rabbit the first cleavage takes place 



about* 22J hours after coitus. It is equal and complete. The 
second cleavage follows in about 3 hours. Here the two cleavage 
spindles frequently lie at right angles to each other so that the four 
blastomeres assume the form of a cross. Cleavage is now irregu- 
lar, 5-, 6-, 7-, and 8-cell stages appearing in order. The 8-cell 


FIG. 54. Cleavage of the monkey's egg. A, first cleavage. B, second cleavage. 
C, third cleavage. X170. (After Lewis and Hartman in Arey.) 

r Vitelline 

Fia. 55. Cleavage of the rabbit's egg. A, fertilized egg (note albumen layer). 
B, first cleavage. C, second cleavage. D, third cleavage. E, fourth cleavage. 
F, fifth cleavage. X180. (After Gregory.) 

stage is attained about 32 hours after coitus. There is now con- 
siderable difference in size, the largest blastomere being almost 
twice the size of the smallest. The 16-cell stage is reached in 
another hour and a half. In reaching this stage the cleavage of 
one blastomere is tangential so that there is always one cell com- 
pletely enclosed. In later cleavages more tangential cleavages 



occur, and this, with the shifting of the blastomeres upon each 
other, results in a solid mass of cells called a morula. 

The blastula. The period of cleavage terminates in the ap- 
pearance of the blastula, but this does not mean that cell division 
comes to an end. The blastula is generally defind^s^_holiQW 
sphere of blastomeres surrounding a cavity, the blastofi^gj. But 

Blastocoel - 

FIG. 56. Diagrams of vertebrate blastulae. A, coeloblastula following holo- 
blastic equal cleavage (amphioxus). B, coeloblastula following holoblastic un- 
equal cleavage (frog). C, discoblastula following meroblastic cleavage (chick). 
D, blastocyst (mammals.) 


this definition does not fit the blastulae formed by meroblastic 
cleavage. So we shall distinguish three classes of blastulae. The 
first is of the hollow sphere type (coeloblastula) and is the result 
of holoblastic equal cleavage '(Fig. 56A). A variety of this type, 
in which the blastocoel is displaced towards the animal pole, is the 
result of holoblastic unequal cleavage (Fig. 56B). 

The second type of blastula (discoblastula) is the result of 
meroblastic cleavage in which the blastomeres rest in a flat disc, 


the blastoderm, on the undivided yolk mass (Fig. 56C). A seg- 
mentation cavity later combines with a yolk cavity, formed by the 
digestion of the yolk underlying the blastoderm, to form a blasto- 
coel. Such a blastocoel is roofed with cells but has a floor of yolk. 

The third type of blastula is found only in mammals and is 
called a blastocyst (Fig. 56D). The solid morula forms a blasto- 
coel which enlarges until it almost separates an outer layer of 
cells (trophoblast) from an inner cell mass (the embryonic knob). 

Presumptive organ regions of the blastula. As might be in- 
ferred from the results of cell-lineage studies, the regions of the 
blastula will give rise to different parts of the embryo in normal 
development. In the tunicate and amphioxus, Conklin has 
mapped out the presumptive organ regions of the blastula, and 
Vogt and his students, by means of a most ingenious technique, 
have accomplished the same result for the amphibian blastula. 
Experimental evidence (Chapter VII) indicates that in the tuni- 
cate and amphioxus the organ-forming regions are definitely de- 
termined whereas in amphibians, the regions have a greater plas- 
ticity and may give rise to parts of the embryo quite different 
from those formed in normal development. 

BLASTULA OF THE AMPHIOXUS. In the development of the 
amphioxus we find a good example of the coeloblastula (Fig. 57). 

The blastomeres are arranged 

Presumptive i i j J.T_ 

ectoderm in a single layer around the en- 

larged blastocoel which is en- 
tirely cut off from the exterior. 
The blastomeres at the animal 
pole are micromeres; those at 
the vegetal pole are macro- 
presumptive meres; the cells at the equa- 
e m torial belt are transitional in 


endoderm The cells which are to form 

FIG. 57. Blastula of the amphioxus. bag- . 

ittal section. X220. (After Conklin.) the mesoderm are rounded and 

in active mitosis. They are 

arranged on a crescent on one side of the egg while those which 
will form the chorda-mesoderm make up a corresponding crescent 
on the other. The endoderm cells are the larger cells of the 
vegetal hemisphere. 





BLASTULA OF THE FROG. The blastula of the frog (Fig. 58) 
resembles that of the amphioxus in all essential characters, but 
shows minor differences due 
largely to the greater amount of 
yolk present. In the first place, 
the blastoderm is no longer one 
layer of cells in thickness. Tan- 
gential divisions have increased 
the number of cells so that at the 
animal pole the blastoderm may 
be approximately four cells deep. 
Furthermore, the greater differ- 
ence in size between the micro- FIG 58 . _ Blastula of the frog . Verti _ 
meres of the animal pole and the C al section. (After Brachet.) 
macromeres of the vegetal pole 

result in the blastocoePs oc- 
cupying an eccentric posi- 
tion entirely within the 
limits of the animal hemi- 

The blastula of the frog 
shows certain regional dif- 
ferentiations. Thus the 
cells of the animal hemi- 
sphere are smaller than 
those of the vegetal hemi- 
sphere. Morgan has point- 
ed out that those arising in 

FIQ. 59. Diagrams of the Triton egg showing the region of the gray Cres- 

movement of surface areas stained with nile cent are definitely smaller, 

blue and neutral red during gastrulation. j dividing more rapidly, 

Areas on surface shown with sharp outline, . i 

than those in any other 

those on interior without outline. 

(After ^nan mose in any 

Vogt has demonstrated 

the fate of different regions of the blastula in normal development 
by marking them with such harmless dyes as nile blue and neutral 
red. The stain persists long enough so that the migration of the 
dyed cell groups can be traced through gastrulation and even later 
(Fig. 59). He has succeeded in mapping out the surface of the 




blastula into presumptive organ regions, as seen in the diagram 
(Fig. 60). 

BLASTULA OF THE CHICK. The blastula of the chick is a dis- 
coblastula. The blastoderm consists of an inner mass of micro- 
meres completely separated from one another by cleavage planes, 
and an outer ring of macromeres which are partially separated 
from one another by incomplete radial cleavage planes only. 
These latter cells are in direct protoplasmic continuity by means 
of an outer ring of undivided cytoplasm and a thin lower layer of 
undivided cytoplasm passing beneath the inner mass (Fig. 61). 
This undivided cytoplasm is called the periblast. The micro- 
meres of the inner mass are separated from the underlying un- 

Blastocoel Blastomeres Periblast 

FIG. 61. Section of early chick blastula. Compare Fig. 53D. (After Patterson.) 

divided periblast by means of a thin cleft which is the original 

The blastoderm expands over the yolk, new cells being added 
to the inner cell mass from the outer ring of cells. The periblast, 
contributing its cytoplasm to the formation of new cells in the 
outer ring, soon uses up all the material contained in the thin 
lower layer. Meantime its outer ring, now called the germ wall, 
expands outward. With the disappearance of the lower layer of 
periblast, the cells of the inner mass form the roof of a cavity 
which includes the original blastocoel plus the space originally 
occupied by the lower layer of periblast. These cells form an area 
known as the area pellucida because it can be detached from the 
yolk without carrying any yolk particles and hence appears more 
transparent. The cells of the outer ring and the germ wall make 
up the area opaca, so-called because particles of yolk adhere to 
them when removed from the egg and render them less trans- 


bryo in the blastula stage has been recorded, so a description of 
the blastocyst of the rabbit will be given in its place. About 75 
hours after coitus and while the egg is still in the oviduct, a cleft, 

the blastocoel, appears in 

, Embryonic knob the morula apparently 

Albumen ^ j onic due to the formation of 
some fluid. This extends 
rapidly until an outer 
layer of cells, the tropho- 
blast, is separated from 
an inner cell mass, the 
embryonic knob. The 
separation is almost com- 

/ , /TV A , x , 

plete (Fig. 62A), extend- 

about 270 of the 

FIQ. 62. Sections of rabbit blastocysts. X200. 

(After Gregory.) possible 360. By this 

time the blastocyst has 

reached the uterus and the secretion of fluid is greatly increased, 
expanding the blastocoel and stretching the trophoblast cells. 
The embryonic knob flattens against one pole (dorsal) of the tro- 
phoblast, and the entire blastocyst increases greatly in size (Fig. 
62B). This flattening of the embryonic knob is not characteristic 
of all mammalian blastocysts. 


The vertebrate blastula becomes converted into a two-layered 
embryo, or gastrula, through the migration of cells from the 
exterior to the interior of the embryo. In so doing the blastocoel 
is obliterated and replaced by a new cavity, the gastrocoel 
(archenteron), which communicates to the exterior by means of an 
opening, the blastopore. The cells left on the exterior form the 
outer germ layer commonly known as ectoderm (ectoblast, epi- 
blast). Those on the inside, lining the gastrocoel, form the inner 
germ layer, usually called the endoderm (entoderm, entoblast, 
hypoblast). But, as will be seen later, they may also include 
cells which will give rise to the middle germ layer, the chorda- 
mesoderm, consisting of the mesoderm (mesoblast) and notochord 
(chorda dorsalis). In such cases the inner layer may be called 



mesendoderm (see page 115). The different types of blastulae 
resulting from different types of cleavage naturally give rise to 
different types of gastrulae (Fig. 63) according to the means by 
which the endoderm is segregated from the ectoderm. 

Students of gastrulation distinguish five types of cell migrations which will be 
described briefly here, and developed more fully in later paragraphs. 

1. Imagination (Fig. 63A). Typical of the cocloblastula resulting from equal 
holoblastic cleavage. The cells of the animal hemisphere move inward in a con- 



Fia. 63. Diagrams of vertebrate gastrulation. A, by invagination (amphioxus). 
B, by epiboly and involution (frog). C, by involution (chick). D, by delamina- 
tion (mammal). 

tinuous sheet, obliterating the blastocoel, until they come to rest against the cells of 
the animal hemisphere, thus giving rise to a new cavity, the gastrocoel, which opens 
to the exterior by means of the blastopore. This process is made possible by the 
continued growth of cells at the lip of the blastopore which roll inward (involution, 
see 3) as invagination continues. 

2. Epiboly (Fig. 63B). Typical of the coeloblastula resulting from unequal 
holoblastic cleavage. The cells of the animal hemisphere grow over the cells of the 
vegetal hemisphere, creating a gradually narrowing circular fold, the lip of the 
blastopore. This process also involves the growth and rolling inward of cells at the 
moving lip (involution, see 3) to form the roof of the gastrocoel. 

3. Involution (Fig. 63B, C). Typical of the discoblastula resulting from mero- 
blastic cleavage. The cells at one region of the disc roll inward and spread out 
under the disc to form the roof of a gastrocoel. The region where involution takes 
place is the dorsal lip of the blastula. Involution also accompanies invagination 
and epiboly (see 1 and 2). 

4. Delamination (Fig. 63D). Typical of the blastocyst in mammals. The lower 



cells of the embryonic knob split off as a loose layer which later reorganizes itself to 
enclose a gastrocoel. 

5. Concrescence (Fig. 64). As the blastopore narrows, cells which originally lay 
along the right and left halves of the dorsal lip converge towards each other. And, 
since the dorsal lip is also growing backward, these cells will form the right and 
left sides of an axial (antero-posterior) streak. 


FIG. 64. Diagrams showing four stages in the process of concrescence, 

(After Lillie.) 

GASTRULATION IN THE AMPHioxus. The first indication of 
gastrulation is a flattening of the macromeres of the vegetal hemi- 
sphere (Fig. 65 A). These cells divide less frequently and become 
more columnar, while the others divide more frequently and be- 
come more cubical or spherical in shape. This change in the 
shape and rate of division, says Conklin (1932), is apparently the 
principal cause of invagination (Fig. 65B), although it may be 
due also in part to the resorption of material from the blastocoel 
jelly, or to exosmosis, for the contents of the blastocoel become 
less viscous as gastrulation proceeds. 

In later stages of gastrulation the gastrula increases in length, 
owing to the backward growth of the lips of the blastopore (Fig. 
65C). While this process is taking place cells are being rolled 
from the exterior to the interior (involution). The lips of the 
blastopore grow unevenly, the ventral lip finally turning upward 



to reduce the blastopore to a very small opening (Fig. 65D). 
Conklin expressly denies that this narrowing of the blastopore is 
caused by the growing together of the right and left halves of the 
dorsal lip (concrescence). The cells left on the exterior after 
gastrulation is complete are ectoderm. Those which have been 
carried to the interior are endoderm, and presumptive chorda- 

presumptive ectoderm 


neural plate 

*V \6' ff 





'Neural plate 

FIG. 65. Sections of amphioxus embryos during gastrulation. A, blastula (6 
hours after fertilization). B, gastrula (9J hours). C, gastrula (12 hours). 
D, gastrula (14 hours). Animal pole indicated by presence of polocyte. Antero- 
posterior axis shown by arrow. All sagittal sections. X180. (After Conklin, 

mesoderm. The segregation of the notochord and mesoderm cells 
is discussed in Section C of this chapter. 

In late gastrulation the cells of the ectoderm develop cilia, by 
means of which the embryo rotates within its fertilization 

GASTRULATION IN THE FROG. The first stage in the gastrula- 
tion of the frog is the formation of a groove on the dorsal side of 
the embryo in the region of the gray crescent (Fig. 66 A). Along 
this groove, cells are pushed into the interior (involution), while 
at the same time the cells immediately above the groove are 
growing down over the surface of the embryo to cover them 
(epiboly). In this way a two-layered fold is produced, the dorsal 
lip of the blastopore (Fig. 66D). 






Roof of 


Neural plate 


Yolk plug 

FIG. 66. Three stages in the gastrulation of the frog's egg. A, dorsal lip stage, from 
vegetal pole. B, do., sagittal section. C, lateral lip stage, from posterior surface. 
D, do., sagittal section E, ventral lip (yolk-plug) stage, from posterior surface. 
F, do., sagittal section. (B, D, F, after Brachet.) 


As the two-layered fold grows down over the cells of the vegetal 
hemisphere, it extends laterally, thus forming the lateral lips of 
the blastopore (Fig. 66B). And, since it is covering a spherical 
surface, the ends of the fold eventually meet to form the ventral 
lip (Fig. 66C). Epiboly and involution take place at all points 
on the lip of the blastopore, but chiefly at the dorsal lip, which 
moves approximately 90 around the egg. At this time the egg 
presents the appearance of a black sphere with a small white 
circular area, known as the yolk plug (Fig. 66C). 

Within the egg, two distinct phenomena have been taking place. 
First, the cells turned inward by involution at the dorsal lip have 
spread out to form the roof of a wide but shallow cavity, the 
gastrocoel. Second, small cells have arisen from the large yolk- 
laden cells of the vegetal hemisphere, and these form the floor of 
the gastrocoel. They join the cells resulting from involution at 
the anterior end of the gastrocoel (Fig. 66E). 

There is now an extensive displacement of the interior cells, re- 
sulting from the growth forward of the gastrocoel, and the conse- 
quent thinning of its floor. It is still uncertain whether the floor 
is pushed across the blastocoel, thereby obliterating it, or whether 
the thin floor is ruptured so that the blastocoel is added to the 
enlarging gastrocoel (Fig. 66F). In either event the center of 
gravity in the egg is now altered so that it rotates about a hori- 
zontal axis in such a way that the blastopore is carried back to a 
point a little beyond its starting point, 100. 

The blastopore is now in its definitive position and marks the 
posterior end of the embryo. The dorsal side, already marked by 
the appearance of the dorsal lip, is uppermost. In the concluding 
stages of gastrulation the blastopore narrows to a small slit. 
This narrowing is brought about by the growing together of the 
right and left halves of the dorsal lip (concrescence) as epiboly 
and involution continue. 

The cells of the inner layer during later stages of gastrulation ap- 
pear to be split into two separate layers. The one of these which 
lines the gastrocoel is endoderm. The other lying between the 
endoderm and the ectoderm is the chorda-mesoderm. The mode 
of origin of the latter will be described in the following section. 


the chick is a disc of blastomeres lying over the undivided yolk. 



Dorsal lip 

It is divided into an interior area pellucida and an outer area 
opaca. This outer area is extending itself in all directions over 

the undivided yolk (epiboly ). 
The account which follows 
is based on gastrulation in 
the pigeon. 

Three zones are distin- 
guishable in the area opaca. 
First, there is a margin of 
overgrowth where the cells 
are completely separated 
from the yolk. Second 
comes a zone of junction, 
whose deeper cells are not 
separated from the yolk. 
The third division is the in- 
ner zone, whose cells, com- 
pletely separate from the 
yolk, are being added to the 
area pellucida. 

The first indication of 
gastrulation is the thinning 
of the blastoderm at the 
posterior end and the com- 
plete separation of the cells 
from the yolk at that region 
(Fig. 67 A) . In other words, 
there is a crescentic area, 
almost a quarter of the cir- 
cumference, of the blasto- 
derm which lacks the zone 

DorsaIlf P of junction completely. 

FIG. 67. Surface views showing three stages jj ere t h e ce u s ro u i nwar d 

in the gastrulation of the hen's egg. from /. , .. x /TV nC i\ i 

the animal pole. (After Patterson.) (involution) (Fig. 68) and 

multiply until they have 

spread completely under the upper layer to roof in the old blasto- 
coel and convert it into the new gastrocoel, whose floor is made 
up of undivided yolk. The slit-like opening where the zone of 

Dorsal Ifp 




junction disappeared is the blastopore, and the rim along which 
involution took place is the dorsal lip. 

There is very little overgrowth at the dorsal lip while involution 
is taking place, and consequently the edges of the blastoderm on 
either side swing around to enclose the lip region in the advancing 

FIG. 68. Sagittal section through early gastrula of pigeon (36 hours after fertiliza- 
tion). Posterior half of section only, d.b., dorsal lip of blastopore. (From 
Richards after Patterson.) 

germ wall. In this way the blastopore is compressed laterally and 
concrescence takes place. 


embryo has been observed before the separation of the germ 
layers. The account which follows is based on the pig. From 
the lower surface of the embryonic knob, individual cells detach 
themselves to form a sheet (Fig. 69) which rapidly establishes 

Kauber's cells 



FIG. 69. Section to show an early stage in the gastrulation of the bat's egg. 

Van Beneden.) 


itself as a layer immediately inside the trophoblast, enclosing 
tnost of the old blastocoel. We may now consider the trophoblast 
and the remainder of the embryonic knob as ectoderm and the 
inner layer as endoderm. The cavity which it encloses is com- 



parable to the gastrocoel plus the yolk sac of the egg-laying 

The cells of the trophoblast immediately overlying the em- 
bryonic knob (Rauber's cells) now disappear, and the embryonic 
knob flattens out to become the embryonic disc. This disc lies 
at the surface and constitutes part of the wall of the blastocyst. 

In the primates, judging from studies on the lemur, Tarsius, 
and from the appearance of the earliest human embryo (Fig. 70), 
the endoderm does not grow out around the entire trophoblast, 





FIG. 70. Diagrams to show three stages in the gastrulation of the human egg 
daring implantation. The uterine wall indicated by hatching. (Hypothetical 
based on Teacher; the embryo in C based on Miller.) 

but forms a very small vesicle immediately under the embryonic 
knob. The cavity of this vesicle may be considered a gastrocoel 
but is more generally known as the "yolk sac." 


During or immediately following gastrulation a third germ 
layer appears between the ectoderm and endoderm. This layer 
consists of the notochord (chorda dorsalis), an axial supporting 
rod found only in the vertebrates and their allies the proto- 
chordates, and two sheets of mesoderm on each side of the noto- 
chord. Later wandering ameboid cells, originating from the 
mesoderm and known collectively as the mesenchyme, make their 

The student should note that in many elementary texts the 
middle germ layer is called the mesoderm and that the notochord 
is variously derived from mesoderm, endoderm (amphioxus and 
frog) , or ectoderm (chick and mammals) . This terminology dates 
back to the phylogenetic period of>embryology (Chapter I), when 


it was supposed that a blastula composed of undifferentiated 
blastomeres gave rise to a gastrula with two separate (primary) 
layers, and that the mesoderm and the notochord arose separately 
from one or the other of the so-called primary layers, primitively 
from the endoclerm. Today it is generally recognized that the 
notochord arises in the same manner and at the same time as the 
mesoderm. To avoid the clumsy phrase, mesoderm and noto- 
chord, many writers are now employing the term chorda-meso- 
derm for the middle germ layer, and restricting the term mesoderm 
to the middle germ layer exclusive of the notochord, a usage 
employed in this text. The compound word mesendoderm 
(mesentoderm) is now used by many writers to include both the 
endoderm and the chorda-mesoderm when these layers lie be- 
neath the ectoderm but have not yet segregated from each 

In collateral reading the student will sometimes encounter the 
word eiido-mesoderm used in connection with mesoderm " origi- 
nating from " or, better, associated with, endoderm in early 
development. Similarly the word ecto-mesoderm is employed 
to designate mesoderm "originating from," or associated with, 
ectoderm in early development. Other writers use the terms 
peristomial mesoderm, meaning mesoderm appearing in the region 
of the blastopore, and gastral mesoderm for mesoderm appearing 
to arise from the invaginated endoderm. But inasmuch as the 
middle germ layer can often be traced to definite blastomeres 
during early cleavage, this distinction is of small importance. 

It is well established, however, that among the vertebrates the 
movement of the presumptive chorda-mesoderm to its definitive 
position in the roof of the gastrocoel is intimately associated with 
the formation and closure of the blastopore. This is true no 
matter whether the blastopore is a large circular opening as in the 
amphioxus and the frog, or reduced *to a primitive streak by 
concrescence as in the chick and man. 

The later history of the germ layers. With the segregation 
of the three germ layers, the presumptive organ regions are now 
located in one or another of the three. But it must not be sup- 
posed that the organs of the adult are exclusively ectodermal, 
endodermal, or mesodermal. On the contrary, most of them con- 
tain material from at least two, and sometimes all three. In 



Part III will be found an account of the development of the differ- 
ent organ systems, classified according to the germ layer from 
which arise the tissues associated with their special functions. 
Meantime the following table is presented. 





A Notochord 
B Mesoderm 

1. Epidermis of skin and 
all openings into the body 
2. Kpithelia of eye, ear, 
and nose 
3. Nervous system, in- 
cluding inter renal glands, 
pituitary gland (in part), 
pineal gland 
4. Epithelium of arnnion 
and chorion 

1 . Epithelium of coelorn 
and exocoel 
2. Nephric (excretory) 
3. Genital (reproduc- 
tive) system 
4. Suprarenal gland 
5. Blood-vascular sys- 
6. Connective tissue 
including skeleton 
7. Musculature 
8. Dennis of skin 

1. Epithclia of diges- 
tive tube, including thy- 
mus gland, thyroid gland, 
parathyroid gland, in- 
ternal respiratory or- 
gans, yolk sac, and al- 


in earlier sections, Conklin (1933) has been able to distinguish the 
mesoderrn cells in the amphioxus in the blastula stage (Fig. 57), 
where they form a crescent of small rounded blastomeres in the 
region where the ventral and lateral lips of the blastopore will 
form. The notochord cells, associated with those which will later 
give rise to the neural plate, occupy a corresponding chorda- 
neural crescent at the dorsal lip. After the invagination of the 
endoderm the cells of the mesoderm and notochord form the lip 
of the blastopore, the notochord cells at the dorsal lip, mesoderm 
at the ventral and lateral lips. As the lips of the blastopore grow 
backward, these cells are carried to the interior by involution 
(Fig. 65). 

When the ventral lip grows upward, the mesodermal cresent is 
tilted up behind so that its arms run in an antero-posterior di- 
rection to form the angles between the roof and sides of the 
gastrocoel (Fig. 71). In the meantime the notochord cells, also 
carried into the interior, form a flat plate between the two arms 
of the mesoderm. Thus the roof of the gastrocoel is composed 



of three strips of chorda-mesoderm, mesoderm on each side, 
notochord in the middle. 

A longitudinal groove in the notochord plate deepens, and the 
folds on either side come together to form a solid cord separate 

Neural plate 




Notochord * 




FIG. 71. Optical hemi-sections of amphioxus gastrula (14 hours after fertilization). 
A, left inside. B, posterior to show notochord and mesodermal groove inside. 
X166. (After Conklin, 1932.) 

from the ectoderm above and the mesoderm on either side. The 
mesodermal grooves (Fig. 72) also become deeper. Transverse 
constrictions meantime appear in the lateral grooves, which 


Level of B- 

FIG. 72. Sections of amphioxus embryo (19 hours after fertilization). A, sagittal 
section. B, transverse section. X166. (After Conklin.) 

divide them into a series of pouches (enterocoels). Finally these 
pouches are constricted off from the gastrocoel and become the 
paired somites (Fig. 73). 

The endoderm, which formerly occupied the floor and anterior 
end of the gastrocoel, extends to form new sides and a new roof. 



The gastrocoel, now for the first time completely lined with 
endoderm, is the primordium of the digestive tube. 

The cells of the chorda-neural crescent remaining on the ex- 
terior of the embryo give rise to the neural plate on the dorsal 
surface. They are covered by the ventral lip of the gastrula as it 
grows over the dorsal side of the embryo. Beneath this covering 

Level of B--{ 


** Enterocoel 

Neural plate 

FIG. 73. Sections of amphioxus larva (24 J hours after fertilization). A, frontal 
section. B, transverse section. X166. (After Conklin.) 

there appears a longitudinal groove with a fold on either side. 
These folds arch up and meet in the ventral line to form the neural 

sections, we owe to Vogt (1929) the identification of the various 
regions on the amphibian blastula. This identification was ac- 
complished by staining small regions of the blastula surface with 
harmless dyes and tracing their movements during and after 
gastrulation (Fig. 59). He finds that the material first to be 
turned in at the dorsal lip is endoderm. Immediately anterior 
and dorsal to this is a crescent-shaped area which will give rise to 
the notochord. On either side of this are the horns of a crescent 
extending from the other side of the blastula which will become 
mesoderm. Immediately anterior to the chorda crescent is the 
crescent-shaped area of the neural plate, the two together being 
equivalent to the chorda-neural crescent of the amphioxus. The 



mesodermal crescent also corresponds to the mesodermal crescent 
of the amphioxus except that its arms already extend dorsally. 

In the gastrulation of the tailed amphibia (urodeles), the mate- 
rial turned in at the dorsal lip is notochord and mesoderm, so that 
the roof of the gastrocoel is chorda-mesoderm as it is in the 
amphioxus, and endoderm cells must grow up from the sides and 
floor to form a new roof. 

In the frog, however, the first material to roll in at the dorsal 
lip of the blastopore is endoderm and notochord (Fig. 60). When 
the material from the meso- 
dermal crescent rolls in, instead 
of following the endoderm, it 
wedges in between the endo- 
derm and ectoderm (Fig. G6F), 
so giving the appearance of 
splitting off from the endoderm 
in the roof of the gastrocoel. 
The roof and sides of the gas- 
trocoel are, therefore, endoder- 
mal except for a narrow dorsal 
strip represented by the noto- 
chord (and a narrow strip be- 
neath it, the hypochord). 
When the notochord (and hypo- 
chord) separate from the roof, 
this small gap is closed by endo- 
derm and the roof is completely 

As the endoderm, notochord, 
and mesodermal regions are 
turned in around the lips of the 
blastopore the overgrowth of 
the lips covers the large yolk- 
laden cells from which the floor 
of the gastrocoel is produced. 
Meantime the expanding cells 
from the ectodermal region of 
the blastula occupy the region 
formerly held by the material which has been turned in. Now the 


FIG. 74. Diagrams showing direction of 
displacements during amphibian gastru- 
lation. A, from posterior surface. B, 
from left side. Thick lines on exterior 
surface. Thin lines on interior. (After 
Vogt, 1929.) 



dorsal lip of the blastopore is the one at which epiboly and invo- 
lution take place most rapidly. Consequently materials on the 
right and left of the mid-dorsal region are stretched towards the 
medial line to take the place of the material lost by involution 
(Fig. 74). In this way the two arms of the mesodermal crescent 
move together to form parallel strips on either side of the noto- 



FIG. 75. Transverse sections to show three stages in the origin of the notochord 
and mesoderm in the frog embryo. (After Brachet.) 

chord. Similarly the two horns of the neural crescent move to- 
gether to form parallel strips which eventually enclose the blasto- 
pore at the posterior end, while the neural plate itself occupies a 
longitudinal dorsal position on the gastrula. All the rest of the 

surface is now material 
which will form the epider- 
mis of the skin. 

The mesoderm continues 
its growth between the ec- 
toderm and endoderm 
(Fig. 75) until it forms a 
continuous sheet except at 
the blastopore. The ma- 
terial on either side of the 
notochord is separated by 

FIG. 76. Diagram of a transverse section of transverse Constrictions 
vertebrate embryo to show the regions of the - f hlook* or snmifp<* pnr 
mesoderm and coelom. 1IltO D1 i ? kS r somites > cor " 

respondmg to the somites 

of the amphioxus. Next comes an intermediate zone from which 
the gonad and kidney will arise. The remainder splits into an 
outside (somatic) layer closely applied to the ectoderm, and an 

Neural tube 



( myocoel ) 


( nephrocoel ) 

_ Lateral 
( coelom ) 




inner (splanchnic) layer similarly applied to the*endoderm. The 
space between (Fig. 76) is the coelom. 

The neural plate develops a longitudinal groove, surrounded at 
the anterior end and sides by ridges known as the neural folds. 
The embryo has now reached the stage known as the neurula 
(Fig. 112). 

tion in the chick takes place after the egg has been laid and incu- 
bation begun. At about the six- 
teenth hour (Patten) the blasto- 
derm is considerably lengthened 
in an antero-posterior direction, 
and has an axial thickening known 
as the primitive streak (Fig. 77). 
This streak represents the dorsal 
lip of the blastopore laterally com- 
pressed through concrescence as 
explained on page 108. The germ 
wall has grown together behind 
the primitive streak and is ad- 
vancing out over the yolk. In a 
more advanced embryo the primi- 
tive streak is differentiated into a primitive groove in the 
middle, primitive folds on either sides, a primitive pit at the 


&; ; . 

r ; 

! *\ 

' ''/'.'''' 
'(-' Y 

" streak 
1 Mesoderm 





FIG. 77. Blastoderm of the chick at 
15 hours of incubation. (After 

'rimitive streak 

Primitive streak 

Endoderm B 


/ Dorsal lip 



A .ff 

FIG. 78. Blastoderm of chick to show early stage in development of notochord. 
A, surface view at 20 hours (after Duval). B, transverse section, left half only. 
C, sagittal section. (B, C, after Lillie.) 

anterior end of the groove, and a primitive (Hensen's) node 
'- front of the pit where the primitive folds unite (Fig. 78). 



Sections reveal that from the sides and posterior end of the 
primitive groove, cells are growing outward, between the ecto- 
derm and the endoderm, to form a sheet of mesoderm. At the 
anterior end a narrow strip of cells grows forward to form the 

During the remainder of the first day of incubation the area 
pellucida increases in length, particularly in the region directly 
in front of the primitive streak. This appears to displace the 
primitive streak rearwards, and during this time the streak 
actually shortens. 

The mesoderm growing out to the sides is carried forward in 
this movement and so comes to lie close to the advancing noto- 
chord. Furthermore, two horns of mesoderm grow forward, later 
to curve in and meet in front of an area which contains ectoderm 
and endoderm only (proamnion). The mesoderm on either side 
of the notochord thickens to form a segmental zone, so called 
because it will shortly be divided by transverse constriction into 
somites, exactly as in the frog. Six pairs of somites are present 
at the end of the first day (Duval). There is a zone of inter- 
mediate mesoderm. The remaining or lateral mesoderm, grow- 
ing out into the area opaca, splits tangentially into an outer 
somatic and an inner splanchnic layer, as in the frog. In the 
splanchnic mesoderm, thickenings appear in the inner region of 
the area opaca. They mark the primordium of the area vasculosa 
(Fig. 79). 

The ectoderm and endoderm of the clear area give rise to a 
crescentic fold at the anterior end which is called the head fold 
as it is the primordium of the head of the embryo. It contains 
a pocket of endoderm known as the fore-gut, distinguished by 
the possession of a cellular floor. There is an opening known as 
the anterior intestinal portal between the fore-gut and the mid- 
gut, whose floor is the undivided yolk. 

The ectoderm in front and to either side of the notochord is the 
neural plate. It develops a groove and folds shortly before the 
end of the first day, and at 24 hours of incubation the folds have 
met in the region of the brain to form a tube but have not as 
yet fused together. 

THE MIDDLE GERM. LAYER IN MAN. The earliest human em- 
bryo is the " Miller " ovum (Fig. 69). This specimen, supposed 



to be about 13-14 days old, consists of an outer vesicle, the 
trophoblast, containing two smaller vesicles, one of which, lined 
with endoderm, represents a small gastrocoel (yolk sac) the 

Zone of junction 
Margin of overgrowth 

FIG. 79. Diagram showing embryonic and extra-embryonic areas of chick embryo 
at 24 hours of incubation. Above, surface view; below, transverse section. 

other of ectoderm surrounds a cavity (the amnion, Chapter V). 
Where the two vesicles are in contact a circular disc of ectoderm 
and endoderm pressed together represents the embryonic disc. 
In later specimens this embryonic disc develops a primitive streak, 





quite as in the chick blastoderm (Fig. 80). Notochord and 
mesoderm develop in much the same way, somites appearing at 

the end of the first month, A 
head fold and neural groove ap- 
-Cut edge of ar j n s j m ii ar fashion, 
amnion ^ 


During cleavage the fertilized 
egg is divided into a large number 
of daughter cells or blastomeres 
which arrange themselves about 
a cavity to form the blastula. 
The pattern of cleavage and the 
form of the blastula vary accord- 
ing to the amount and distribution 
FIG. 80. Surface view of embryonic of the yolk in the fertilized egg. 

disc in human embryo after amnion The presumptive organ reg ions of 
has been cut away. X40. (After ., r ,... ^ , T 

Heuser.) the fertilized egg are segregated 

into different groups of cells which 
compose the presumptive organ regions of the blastula. 

During gastrulation, the blastomeres are reorganized into differ- 
ent strata or germ layers about a new cavity, thus forming a 
gastrula. The method of gastrulation varies according to the 
type of blastula formed after cleavage. The two layers segre- 
gated during gastrulation are usually known as the ectoderm 
and endoderm, but it must be recognized that one or the other 
of these so-called primary layers includes the presumptive 
mesoderm as well. 

In the concluding period of germ-layer formation, the middle 
germ layer or chorda-mesoderm, including the notochord and the 
mesoderm proper, is segregated from the other germ layers to 
occupy a middle position between them. 

While the germ layers are being segregated from each other the 
primordia of certain organs are arising from their respective pre- 
sumptive regions. Thus the notochord is separated from the 
mesoderm proper, the neural plate from the presumptive epi- 
dermis. In the mesoderm proper, the somites begin to take form, 
and the somatic layer separates from the splanchnic to form the 



Brachet, A. 1921. Traite* d'embryologie, Books 3, 4, and 5. 

Conklin, E. G. 1905. The Organization and Cell Lineage of the Ascidian Egg. 
Jour. Acad. Nat. Sci. Phila., 2nd Series, Vol. XIII. 

1932. The Embryology of Amphioxus. Jour. Morph. 54:69-151. 

1933. The Development of Isolated and Partially Separated Blastomeres of 

Amphioxus. Jour. Exp. Zool. 64:303-375. 

Cowdry, E. V. (ed.) 1924. General Cytology, Section 9. 

Gregory, P. W. 1930. The Early Embryology of the Rabbit. Publ. Carnegie 
Inst. Wash. 407:141-168. 

Hertwig, O. (ed.) 1906. Handbuch, etc., I, Chaps. 2 and 3. 

Huxley, J. 8., and de Beer, G. R. 1934. The Elements of Experimental Embryol- 
ogy, Chap. 2. 

Jenkinson, J. W. 1913. Vertebrate Embryology, Chaps. 5 and 6. 

Kellicott, W. E. 1913. General Embryology, Chaps. 7 and 8. 

Kerr, J. G. 1919. Textbook of Embryology, II, Chap. 1. 

Korschelt, E., and Heider, K. 1902-1910. Lehrbuch, etc., Chaps. 7 and 8. 

Lillie, F. R. 1919. The Development of the Chick, 2nd Ed. 

MacBride, E. W. 1914. Textbook of Embryology, I, Chap. 17. 

Patten, B. M. 1929. The Early Embryology of the Chick, 3rd Ed. 

1931. The Embryology of the Pig, 2nd Ed. 

Wilson, E. B. 1925. The Cell, etc., Chaps. 13 and 14. 



After the germ layers have been segregated, the primordia of 
several great organ systems are already localized. Before pro- 
ceeding to an account of the way in which the organ systems de- 
velop from the different germ layers (organogeny), we must 
examine the way in which the vertebrate body assumes its form. 
This is found to be closely connected with certain structures 
(adnexa) which develop also from the germ layers and play an 
important part in embryonic (and fetal) life, but which are dis- 
carded before hatching (or birth). These extra-embryonic struc^ 
tures are the yolk sac, the amnion, chorion, and allantois, as well 
as a structure found only in the mammals, the placenta. 


The general form of the vertebrate body is cylindrical, while 
the form of the vertebrate egg is spherical. There are in general 

two methods of growth 

^ by means of which the 

cylindrical shape is at- 
tained. In the first, 
characteristic of small- 
yolked eggs with a 
spherical gastrula, the 

FIG. 81. Diagrammatic transverse sections show- *n&lH lactor IS growtn in 
ing effects of yolk on form of embryo. A, small length, along the antero- 
yolked embryo (frog). B, large yolked embryo posterior (cephalo-Cau- 
(chick). (After Assheton.) i i\ T ii i 

dal) axis. In the second 

type, which is characteristic of large-yolked eggs, the embryo is 
modeled from a flat disc into the form of a cylinder connected with 
a great yolk sac by some sort of pedestal or stalk. Much of this 
modeling is done by the outgrowth of the head and the tail re- 
spectively, especially among the anamniote vertebrates, but there 



is also some actual undercutting, especially evident among the 
Amniota. This undercutting is accompanied by the formation 
of amniotic folds, as will be seen in the development of the chick. 
A diagram of cross-sections through the body of a small-yolked 
embryo (Fig. 81A) and a large-yolked embryo (Fig. 81B) will 
make clear the difference between the cylindrical embryo and the 
plate-like embryo before it has been remoulded. In the amniote 
vertebrates with a large-yolked egg the embryo develops from 


FIG. 82. Diagrams to show growth in length by concrescence. Arrows indicate 
direction of growth. (After Assheton.) 

material at the edge of the blastoderm, and as this is rolled together 
in concrescence the embryo increases in length (Fig. 82). 
/* General plan of the body. The body of the vertebrate is 
basically a tube within a tube, i.e., a digestive tube within a body 
tube (Fig. 76). 

The digestive tube is endodermal in origin and originates from 
the gastrocoel. Here again the small-yolked form has a tubular 
intestine from the beginning. It is only necessary to form an- 
terior and posterior openings, for the blastopore either closes or 
is roofed in by the neural folds. The new openings arise from 
ectodermal pits, the stomodgum at the anterior end, the procto- 
deum at the posterior end. In general these openings are not 
completed until after the yolk has been wholly consumed. The 
gastrocoel of large-yolked embryos has only a roof and sides of 
endoderm, for the floor is composed of the yolk. Hence the roll- 
ing in or undercutting of the body commencing at the head end, 
and later at the tail end, forms a pocket at each end, the fore-gut 
and hind-gut respectively. The mid-gut is the remainder of the 
open gastrocoel connected with the developing yolk sac by means 
of the yolk stalk. 


Between the two tubes lies the mesoderm. The ventral meso- 
derm of small-yolked embryos (lateral of large-yolked forms) 
splits into a somatic and splanchnic layer. The first of these is 
closely applied to the ectoderm to form the somatopleure ; the 
second is associated with the endoderm to form the splanchno- 
pleure. The space between is the coelom or body cavity. Other 
and lesser antero-posterior tubes such as the neural tube, formed 
from ectoderm, and axial blood vessels, e.g., the aorta, formed 
from mesoderm, are indicated in the figure and will be discussed 
in later chapters. 

Metamerism. With growth in length is associated a second 
factor in the development of the vertebrate body, that of metam- 
erism. This is first indicated by the appearance of metameres 


..Nasal pit 

Visceral clefts 

Oral gland 




FIG. 83. Diagrams of early embryos to show similarities in body form. A, frog 
(after W. Patten). B, chick (after Kerr). C, man (after His). 

such as the enterocoels in the amphioxus or somites in the true 
vertebrates. In later organogeny are found further evidences of 
metamerism in the nervous system, nephric system, vascular sys- 
tem, and others. However, the primary metamerism of the body 
is shown in the mesoderm. The somites are formed successively, 
commencing at the anterior end and therefore affording a basis 
of classifying the early embryos of any species by the number of 
these units present (Fig. 83). 


The head. The vertebrate body is distinguished by a well- 
marked region at the anterior end, containing the mouth, visceral 
arches, special sense organs (nose, eye, and ear) and the highly 
developed brain. Herein the amphioxus differs from the verte- 
brates, for it has so little head that some zoologists make a special 
group (Acraniata) to contain it. 

The anterior end of the body is already determined in the verte- 
brate egg (animal pole). It is the surface opposite that of the 
blastopore, or in front of the primitive streak. It is the region 
where the neural folds first arise and where they first meet. It 
is the first part of the body to be freed of the yolk in the large- 
yolked embryos. A glance at the diagrams of early embryos 
(Fig. 83) will suffice to prove that this is the most highly differen- 
tiated part of the body. 

In the Amniota the head is inclined ventrally at the region of 
the branchial arches. This cervical flexure causes a constriction 
(Fig. 83) which is the primordium of the neck, a region found only 
in reptiles, birds, and mammals. 

The tail. All vertebrate embryos, even those of species in 
which the adult is tailless (frog, man), develop a well-marked tail 
in early development. This region is characterized by the ab- 
sence of a digestive tube and coelom. It develops early in the 
anamniotes, where it is of great use to the free-swimming larva, 
but more slowly in the amniotes. 

The appendages. The paired appendages of vertebrates arise 
as buds (Fig. 83C) which later develop into fins or limbs. Limb 
buds do not appear in the amphioxus or the cyclostomes. In all 
other vertebrates which do not possess paired appendages in the 
adult condition, it is said that limb buds appear in the embryonic 
life and are resorbed later. 

BODY FORM OF THE FROG. The spherical egg of the frog, 
being only moderately telolecithal, is converted into the cylindri- 
cal shape of the embryo principally through the growth of the head 
and of the tail. 

In the head region the neural plate is much wider than else- 
where, and when the neural folds close in to form the neural tube 
the brain will be larger than the spinal cord. On either side of 
the head the optic vesicles, the primordia of the eyes, push out 
from the brain and make well-marked bulges. The ectoderm im- 


mediately external to each optic cup will later give rise to the lens 
of the eye. Anterior to each eye is a depression in the ectoderm, 
the nasal (olfactory) pit. These pits are the primordia of the 
nose. Posterior to each eye a similar otic (auditory, acoustic) 
pit originates, the primordium of the inner ear. On the ventral 
side, folds of ectoderm give rise to the ventral sucker (mucous 
gland) in the form of the letter V. Between the limbs of the V 
there appears an ectodermal pit called the stomodeum or primor- 
dium of the mouth. On the ventral side of the body, just ante- 

FIG. 84. Growth of the frog embryo. A, late neurula, 2.4 mm. B, embryo of 3 
mm. C, embryo of 6 mm., just hatched. D, young larva, external gill stage, 
9 mm. E, larva, internal gill stage, 11 mm. (Measured alive and drawn after 
preservation. X10.) 

rior to the base of the tail, a similar pit, the proctodeum, is the 
primordium of the cloacal opening. 

On the sides of the head five dorso-ventral grooves appear 
(in the order I, V, II, III, IV). These are the visceral (branchial, 
" gill ") grooves, some of which will later break through into cor- 
responding outpushings from the fore-gut, the visceral (pharyn- 
geal, " gill ") pouches, to form the visceral (pharyngeal, " gill ") 
clefts. For the present we need simply note that they separate 
six transverse bars or ridges which are known as the visceral 



arches. Each visceral arch contains an aortic arch. (See Table 
8.) Arch I (mandibular) contributes to the formation of the 
jaws. Arch II (hyoid) contributes to the gill cover (operculum) 
and to the support of the tongue. Arches III, IV, and V are often 
known as branchials 1, 2, and 3, respectively. On arches III, 
IV, and V develop outgrowths which become the external gills 


( From cndoderm ) 




(From ectoderm) 

arch I 

arch I 

pouch I 

cleft I 
(spiracle of 

groove I 

arch II 

arch II 

pouch II 

cleft II 

groove II 

arch 111 
(1st branchial) 

arch III 

pouch III 

cleft III 

groove III 

arch IV 
(2nd branchial) 

arch IV 

pouch IV 

cleft IV 

groove IV 

arch V 
(3rd branchial) 

arch V 

pouch V 

cleft V 

groove V 

arch VI 
(4th branchial) 

arch VI 

pouch VI 
(vestigial in frog) 

cleft VI 
(lacking in frog) 

groove VI 
(lacking in frog) 


(branchiae). That on V is rudimentary. Later a fold grows 
from arch II to cover the external gills completely on the right, 
but with an opening on the left known as the atriopore (" spir- 
acle"). While this is taking place the grooves between arches 
II, III, IV, V, and VI break through into the corresponding 
visceral pouches to form the visceral clefts. Internal gills (demi- 
branchs) develop in the clefts, and the external gills disappear. 
Meantime the mouth has opened and developed horny jaws. 

The tail arises by the backward growth of the tissue in the neural 
folds (Bijtel) at the point where they united over the blastopore. 
The notochord and neural tube grow backward, carrying epi- 
dermis and muscle-forming material with them. Dorsal and 
ventral folds make the tail fin. 

The paired limbs arise as limb buds. The anterior buds arise 
first but are concealed beneath the operculum. The one on the 
left side appears first, pushing through the atriopore. 

BODY FORM IN THE CHICK. The body of the chick is cut off 
from the blastoderm by the outgrowth of a head fold accompanied 
by an undercut, the subcephalic pocket, which appears during 
the first day of incubation. This fold extends backward in the 
form of an inverted U as the lateral folds arise. These are 
also accompanied by undercuts known as the lateral sulci. 
Finally there is a posterior tail fold accompanied by a subcaudal 
pocket appearing on the third day. Outgrowth at the folds with 
some undercutting as well causes the body of the embryo to stand 
up from the surrounding blastoderm to which it is attached by a 
short pedestal, the umbilical stalk. The head bends down sharply 
at the cephalic flexure, but pressing against the yolk, it turns 
or twists toward the right so that the left side of the head rests 
on the yolk. The ventral bend is known as flexure, the dextral 
twist is known as torsion. Flexure and torsion commence in the 
middle of the second day of incubation, and continue in a caudal 
direction until, at the end of the fourth day, the chick lies com- 
pletely on its left side. 

The primordia of the brain and sense organs arise much as they 
do in the frog. A stomodeum appears early in the third day of 
incubation, the proctodeum during the fourth day. Four visceral 
grooves (in the order I, II, III, IV) and five arches appear between 
the end of the second and beginning of the fourth day of incuba- 




C D 

FIG. 85. Growth of the chick embryo. A, 25 hours of incubation. B, 38 hours of 
incubation. C, 48 hours of incubation. D, 08 hours of incubation. Compare 
Figs. 200, 206, 212, 218, respectively. A, B, approx. X9; C, D, approx. X4. 
(After Duval.) 


tion. Only the first three clefts actually open into the fore-gut, 
and these are soon closed again. 

The tail arises from the backward growth of the tail fold but 
never attains any great length. 

The limb buds appear during the third day of incubation. 

BODY FORM IN MAN. Human embryologists distinguish three 
periods during intra-uterine development: the period of the 
ovum, from fertilization to germ-layer formation, two weeks; 
the period of the embryo, until the embryo has assumed a defi- 
nitely human appearance, the end of the second month; and 
the period of the fetus. It is the second of these with which we 
are concerned. 

By the end of the third week the head fold is formed, and at the 
fifth the tail fold is developed. Neural folds are formed and unite 

A B C 

FIG. 86. Growth of the human embryo. A, neural folds (after Ingalls). B, neural 
tube commencing, seven somites (after Payne). C, ten somites, (after Corner). 

much as in the chick (Fig. 86). The primordia of eye, ear, and 
nose-also ^originate in a similar manner. Five visceral grooves 
are formed, by the end of the fifth week, separating six visceral 
arches, but although the visceral pouches appear and unite with 
the grooves, true visceral clefts are not formed. By the end of 
the seventh week, the visceral grooves have disappeared. A 
cephalic flexure appears in the fifth week. The neck (cervical) 


flexure develops in the week following and accelerates the dis- 
appearance of the visceral grooves. 

A tail is developed from the tail fold which is quite prominent 
during the six and seventh weeks of development but is overgrown 
and resorbed during the eighth. 

Limb buds make their appearance toward the end of the fifth 


Yolk sacs are found in the development of all large-yolked eggs, 
among both anamniotes and amniotes. As the name implies, 
this structure is a larger or smaller bag protruding from the body 
and connected with it by a yolk stalk. 

Origin and development. The yolk sac develops from the 
outer margin of the blastoderm which advances under the vitel- 
line membrane and around the yolk mass until the yolk is com- 
pletely enclosed (Fig. 82). 

Function and fate. It contains the yolk, which, in mero- 
blastic cleavage, is not divided among the blastomeres. But it 
plays a far more important part in development than simply 
acting as a reservoir for food reserves. (It is lined with endoderm 
just like that of the intestine, and is furnished with arteries, veins, 
and capillaries, which make up the area vasculosa. The endo- 
dermal lining digests the yolk, and the vitelline veins carry the 
digested food to the developing embryo. We may think of the 
yolk sac as an extra-embryonic intestine. ) It is interesting to note 
that in some viviparous elasmobranchs, like the dogfish, the yolk 
sac continues to be of use, even after the yolk is consumed. 
Pressed against the wall of the uterus it absorbs the uterine 
" milk " which this organ secretes (much like a tertiary egg 
envelope) and conveys it to the embryo through the vitelline 
veins. A similar device is seen among the marsupials (page 144). 
The yolk sac is usually drawn up into the body when the umbilicus 
closes and is later resorbed.^ 


no yolk sac, for the yolk is divided among the large blastomeres 
which later make up the floor of the intestine. The mass of these 
cells, however, creates a bulge on the ventral surface of the em- 
bryo (Fig. 84) which resembles externally a small sac. 


THE YOLK SAC OF THE CHICK. The yolk sac of the chick is 
formed by the advancing edge of the blastoderm. Looking down 
on the blastoderm of the chick at the end of the first day of incu- 
bation (Fig. 79), one distinguishes a series of concentric rings. 
Proceeding from the periphery inward, we note first the area vitel- 
lina externa, consisting of the margin of overgrowth and the zone 
of junction (page 112). Then comes the area vitellina interna in 
which we can distinguish the ectoderm and endoderm, the latter 
closely applied to the yolk. Finally there is distinguished the 
area vasculosa into which the mesoderm has pushed, splitting, as 
it advances, into the somatic layer (next the ectoderm) and the 
splanchnic layer (next the endoderm). Between the somatic and 
splanchnic layers lies the exocoel (extra-embryonic coelom), as 
the coelem is called when it extends beyond the boundaries of the 
embryo. The blood vessels of the area vasculosa develop in the 
splanchnic mesoderm. The exocoel separates the splanchno- 
pleure (endoderm and splanchnic mesoderm) from the somato- 
pleure (somatic mesoderm and ectoderm), so that it can be said 
that the yolk sac of the chick consists of splaiichnopleure. By the 
end of the fourth day of incubation the yolk is completely cov- 
ered except for a small area at the vegetal pole, known as the 
yolk sac umbilicus (Fig. 89C, D). When the chick hatches, the 
empty yolk sac still attached to the intestine is drawn into the 
coelom and gradually disappears. 

THE YOLK SAC OF MAN. In man, as in other mammals, the 
yolk sac arises in connection with gastrulation. The endoderm 
growing out from the lower surface of the embryonic knob ap- 
parently reorganizes itself to form a very small gastrocoel or yolk 
sac. The roof of this gastrocoel forms the roof of the digestive 
canal; the anterior end is set off (with the head fold) to make the 
fore-gut; the posterior end is set off (with the tail fold) to make 
the hind-gut. The remainder constitutes the small yolk sac 
(Fig. 86 A). This sac is later squeezed between the amnion and 
chorion (Fig. 90), and loses its connection with the intestine, 
through the degeneration of the yolk stalk. 

In other mammals (Fig. 68) the endoderm grows completely 
around the interior of the trophoblast and forms a larger yolk sac. 
In the mouse, where the embryonic knob hangs well down in the 
cavity of the blastocyst, this results in the knob's being covered 





FIG. 90. Diagrams to show development of extra-embryonic structures in human 
embryo. Four stages illustrated by sagittal sections. (After Corning.) 


Rauber's cells 




Fia. 91. Amnion formation in the bat's egg. A, primary amniotic cavity. B, 
origin of amniotic folds. (After Van Beneden.) 


ate. The embryonic disc thus comes to form part of the 
blastocyst wall. 

The amnion and chorion are formed by amniotic folds (Fig. 91). 
The internal limb of each fold is formed of somatopleure derived 
from the embryonic disc and will form the amnion as in the chick. 
The outer limb of each fold, however, is formed of ectoderm 
derived from the trophoblast associated with somatic mesoderm 
and gives rise to the chorion. The mesoderm growing out from 
the primitive streak, and delaminating into somatic and splanchnic 
layers, becomes the lining of the exocoel. 


The development of an amnion and chorion is always accom- 
panied by the appearance of another sac, the allantois. This 
extra-embryonic structure appears as an evagination from the 
hind-gut and is therefore lined with splanchnopleure. It grows 
out through the exocoel of the umbilical stalk into the exocoel of 
the chorion, which it usually fills. It is filled with an allantoic 
fluid which receives the nitrogenous wastes of the embryo in the 
form of uric acid (Needham), and may be thought of in the first 
instance as an extra-embryonic urinary bladder. As it fills the 
chorion, its walls, being composed of splanchnic mesoderm in the 
outer layer, easily fuse with the mesodermal layer of mesoderm 
of the amnion, chorion, and yolk sac, whenever these structures 
come together. Furthermore, it has an area vasculosa served by 
the allantoic (umbilical) veins and arteries. This area vasculosa 
when applied to the chorion is the region where the blood is nearest 
to a source of atmospheric oxygen. Here an exchange of gases, 
carbon dioxide for oxygen, takes place, and the allantois may be 
considered as an extra-embryonic lung. 

In the cleidoic egg of reptiles, birds, and egg-laying mammals, 
the allantois also takes part in the formation of an albumen sac 
wherein this material is digested. In the marsupials and placental 
mammals it contributes to the formation of a placenta (hemi- 
placenta in marsupials) whereby digested food is obtained from the 
maternal circulation. These functions of the placenta will be 
discussed in the sections following. 

ALLANTOIS OF THE CHICK. The allantois (Fig. 92) arises to- 
wards the end of the third day as an evagination from the floor of 



the hind-gut. It grows out between the yolk and the wall of the 
subcaudal pocket into the exocoel (Fig. 89B). Here it expands 
greatly until by the end of the ninth day it has filled the entire 
exocoel. Its outer wall unites with the chorion (Fig. 89C) to form 
a chorio-allantois, its inner wall unites with the amnion above 
and the yolk sac below. 

Now the chorion, carrying with it an inner fold of allantois, 
grows down beyond the yolk-sac umbilicus (page 136), and around 



Yolk sac 



FIG. 92. The embryo chick and its extra-embryonic structures on the sixth day of 
incubation. XI 2- (After Duval.) 

the mass of albumen, which has become more viscous through the 
loss of water and is displaced towards the lower side of the egg. 
The albumen is enclosed in a double-walled sac of chorion with 
the allantois between the two walls of the sac (Fig. 89D). The 
layer next to the albumen is the ectoderm of the chorion, but 
the mesoderm of the allantois supplies the blood vessels. It is 
interesting to observe that it is the ectoderm of the albumen sac 
which absorbs the albumen, whereas in the yolk sac it is the 
endoderm which carries on this function. 

By the twelfth day of incubation the albumen sac is closed 
except at the yolk-sac umbilicus where it has an open connection 


with the yolk sac. On the sixteenth day the albumen is consumed. 
On the seventeenth the yolk-sac umbilicus closes by the constric- 
tion of a ring of mesoderm derived from the old edge of the blasto- 
derm. The yolk sac with the remains of the albumen sac still 
attached is retracted into the body cavity of the chick on the 
nineteenth day of incubation, aided by contractions of the amnion 
and the inner wall of the allantois. 


mammals there is a well-developed allantois, arising like that of 
the chick, growing into the exocoel, and uniting with the chorion 
to participate in the formation of the placenta, but the human 
allantois is rudimentary. It arises as a minute tubular evagina- 
tion which develops from the endodermal roof of the gastrocoel 
even before the formation of the tail fold. It grows out into the 
body stalk, a mass of mesoderm connecting the embryo with the 
chorion (Fig. 90) for a short distance, but never gets so far as the 
chorion. However, the allantoic (umbilical) blood vessels con- 
tinue down the body stalk to the chorion where they form a chori- 
onic area vasculosa in the region of the developing placenta. 


Before discussing the human placenta it will be helpful to re- 
view the different types of placentation recognized in mammals. 
Two types are distinguished according to the degree of union be- 
tween the trophoblast and the lining of the uterus (mucosa); a 
second basis of distinction is whether the wall of the allantois 
comes in contact with the chorion or not. 

Indeciduate type. The first type of placenta is called inde- 
ciduate. In this type, found in several groups particularly the 
ungulates, the trophoblast is closely applied to the mucosa but 
both retain their integrity. The blood vessels of the placenta 
absorb food material excreted by the mucosa and exchange carbon 
dioxide for oxygen by diffusion. 

Marsupials. Among the marsupials are found both non- 
allantoic and allantoic hemiplacentae. In the opossum, Didel- 
phys (Fig. 93 A), the enlarged yolk sac is pressed against the 
trophoblast, which in turn is closely applied to the mucosa, 
forming folds which project into depressions in the uterine wall. 
The absorbed nutriment is conveved to the embrvo bv means of 



the area vasculosa of the yolk sac. In Perameles (Fig. 93B), an 
allantoic hemiplacenta is formed by the union of the allantoic 
sac with the trophoblast. Where this hemiplacenta touches the 

Yolk sac - 
A B 

FIG. 93. The extra-embryonic structures of marsupials. Diagrammatic. 
Didelphys. B, Perameles. (After Jenkinsoii.) 


mucosa the epithelium of the latter thickens and is invaded by 
maternal capillaries. The trophoblast is said to be resorbed so 
that the capillaries of the allantois come into intimate connection 
with those of the uterus. It should be mentioned in this connec- 
tion that Perameles also possesses a well-developed area vasculosa 

Allantoic cavity 



FIG. 94. Diagram of extra-embryonic structures in the pig. (After Smith.) 

on the yolk sac. It is very probable, therefore, that both yolk sac 
and allantoic circulations are concerned with the nutrition of the 
developing young. 

Ungulates. In the ungulates there is a well-developed allan- 
toic placenta of the indeciduate type (Fig. 94). The blastocyst 
elongates, and over its surface appear projections of the tropho- 


blast which contain a core of mesoderm. These projections, villi, 
grow into corresponding depressions of the uterine wall, called 
crypts. The allantois meantime has filled the exocoel, and capil- 
laries from the allantoic arteries and veins penetrate the meso- 
dermal cores of the villi. These capillaries are brought very near 
those of the uterine wall, but the blood remains separated from 
that of the mother by (1) the endothelial lining of the maternal 
capillaries, (2) the connective tissue of the mucosa, (3) the 
epithelium of the mucosa, (4) the trophoblast, (5) the mesoderm 
of the villi, and (6) the endothelial lining of the fetal capillaries 
(Fig. 99A, B). At birth the villi are pulled out of the crypts, and 
the placenta, with the remaining embryonic membranes, is dis- 
charged as the " after-birth. " 

Deciduate type. The second type of placentation is called 
deciduate. In this type the trophoblast attacks the mucosa and 



Wall of uterus 

FIG. 95. Diagram of extra-embryonic structures in the dog. Sagittal section. 

(After Jenkirison.) 

erodes part of the lining. It is characteristic of the majority of 
the clawed mammals (unguiculates) and primates. In the first 
group the placenta is allantoic ; in the primates, non-allantoic. 

Carnivores. In the carnivores (Fig. 95) is found a deciduate 
placenta of the allantoic type. The blastocyst elongates although 
not to the extent it does in the ungulates. During this time 


the epithelium of the uterus is cast off. At the circular zone of 
the uterus which is in contact with the equator of the blastocyst 
the epithelium of the uterus fails to regenerate. Into this 
zonary area grow the villi of the trophoblast which become pene- 
trated by the allantoic capillaries. The villi send out branched 
processes, each with its capillaries, which surround the maternaf 
capillaries. Thus the maternal blood is separated from that of 
the fetus by (1) the endothelium of the maternal capillaries, (2) a 
varying amount of maternal connective tissue, (3) the tropho- 
blast, (4) a varying amount of chorionic connective tissue, and 
(5) the endothelial lining of the fetal capillaries (Fig. 99C). At 
birth a certain amount of maternal tissue is torn away with the 

PLACENTA OF MAN. In the human placenta there is the most 
intimate contact between the maternal and fetal circulation. 

cavit y m (!) fit ) Allantoic cavity 



FIG. 96. Diagram of extra-embryonic structures in man. (After Kolliker.) 

The placenta is non-allantoic. It will be recalled that the em- 
bryonic knob retains its connection with the trophoblast as the 
body stalk. Into the body stalk grows the small evagination 
from the hind-gut which represents the endodermal lining of the 
allantois (Fig. 90). It -never comes in contact with the tropho- 
blast and soon degenerates. The limiting sulci of the amnion 
approach each other and become the walls of the umbilical cord. 




basal is 

This encloses (Fig. 96) the body stalk, yolk stalk, allantoic stalk, 

as well as the two umbilical arteries and two umbilical veins which 

grow out from the body 
of the embryo towards 
the trophoblast. These 
umbilical blood vessels 
represent the allantoic 
vessels of all other am- 
niotes. Later the um- 
bilical veins fuse, and 
all this tissue assumes 
common connective tis- 
sue characteristics ^with 
the exception of the 
walls, of the blood ves- 

The deciduae. It 
will be remembered that 
the blastocyst burrows 
into the uterine wall, 
eroding epithelium, con- 
nective tissue, and blood 

vessels. As the embryo increases in size, this erosion continues 

and the embryo sinks into the compact layer of the mucosa and 

comes in contact with the 

spongy layer. The mucosa 

grows around the burrowing 

embryo, shutting it off from 

the cavity of the uterus. 

There may now be distin- 
guished (Fig. 97) three 

regions in the mucosa: (1) 

the decidua basalis, to which 

-:.* ^'/x^OTvr-^itscjy^^N... .. 

the blastocyst is attached; 
(2) the decidua capsularis, 

which cuts off the blastocyst F - 98. Human embryo 11 mm. in^length, 

from the uterine cavity; 
and (3) the decidua vera, 
including the remainder of the uterine lining. 

Fia. 97. Diagram to show the uterine deciduae 
(human). (After Kollmann.) 





yolk sac 

about 6 weeks old, to show extra-embryonic 
structures. X 1 2 . (After Arey . ) 



The chorion. The trophoblast, while entering the uterine 
wall, becomes differentiated into an outer syncytial layer and an 
inner cellular layer. During the process of implantation, nutri- 
tion is obtained by the syncytial layer, which sends out projections 
or false villi into the maternal tissue. Thereafter mesodermal 
cores grow into the false villi converting them into the true villi 
which later receive capillaries from the umbilical blood vessels. 




epithelium &L \ 

A\ A 

Uterine /j 
capillary J^' 




~2~r Chorionic 


FIG. 99. Sections through placentae of A, pig; B, cow; C, cat; and D, human. 
(Semi-diagrammatic after Grosser.) 

Some of these bore into the uterine wall to become fixation villi. 
The others, losing their syncytial layer, remain in the space be- 
tween the trophoblast of the chorion and the maternal tissue as 
nutrition villi (Fig. 98). These are bathed in maternal blood 
which is brought into the intervillous space and carried thence 
by the eroded uterine capillaries. Only those villi which are in 


contact with the decidua basalis persist; the others degenerate, 
thus differentiating the chorion into the chorion frondosum, with 
villi, and the chorion laeve, devoid of the same. In the huifcan 


Villi of 




vera and 


FIG. 100. Diagram of fetus (near term) to show relationships of extra-embryonic 
structures and deciduae. (After Ahlfeld.) 

placenta the maternal blood is separated from the fetal blood 
stream by only (1) the cellular layer of the trophoblast, (2) the 
chorionic connective tissue of the villi, and (3) the endothelia of 
the fetal capillaries (Fig. 99D). 


Parturition. The history of the extra-embryonic structures 
as well as that of the deciduae is terminated by birth (parturition). 
Owing to the absence of an allantoic sac the amnion enlarges to 
fill the exocoel. Later, growth of the fetus results in pressing the 
chojion laeve and decidua capsularis against the decidua vera and 
ooDJ^rating the uterine cavity (Fig. 100). At birth the placenta, 
carrying with it the decidua basalis, and the attached membrane, 
wKfch represe|pf the fused amnion, chorion laeve, decidua capsu- 
laris, and deciihia vera, are cast off as the caul or "after-birth." 

' t '' 


The method by which the external form of the vertebrate 
embryo is assured is closely connected on the one hand with the 
shape of the gal||iila, and on the other with the presence or ab- 
sence of certain extra-embryonic structures, the yolk sac, amnion, 
chorion, and allantois. 

With growth in length we associate the occurrence of metamer- 
ism, or the serial repetition of parts, and the formation of a head 
and a tail. The paired limbs arise as buds. 

The yolk sac is found only in embryos developing from ex- 
tremely telolecithal eggs. It is lined with endoderm and func- 
tions as an extra-embryomc intestine. The splanchnic layer of 
the mesoderm adjacent to it develops an area vasculosa which 
conveys the digested yolk to the body of the embryo. 

The amnion and the chorion arise typically from folds of somato- 
pleure which fuse above the embryo, thus giving rise to an inner 
membrane, the amnion, and an outer one, the chorion. The 
amnion, lined with ectoderm internally, contains amniotic fluid 
in which the embryo develops. The chorion, lined with somatic 
mesoderm internally, contains the exocoel, a continuation of the 
embryonic coelom. Neither of these membranes has any vascular 
system of its own. They are found only in the development of 
reptiles, birds, and mammals. 

The allantois always develops in amniote embryos. It arises 
as a ventral evagination of the hind-gut and typically grows out 
into the exocoel which it completely fills. It functions as an 
extra-embryonic bladder and lung, and because of its vascular 
area may act (in connection with the chorion) as an organ of 
nutrition, e.g., as an albumen sac. 


In mammals the blood vessels of the allantois invade the 
chorion giving rise to the placenta, an organ where substances 
may be exchanged by diffusion between the maternal and fetal 
blood streams. The placenta is connected to the embryo by the 
umbilical stalk, whose walls are formed by the amnion. In some 
mammals, such as the carnivores and primates, partfe of the 
uterine wall, the deciduae, are concerned in the formation of the 
placenta, and cast off with them at birth. 

" ~\ 

Allen, E. (ed.) 1932. Sex and Internal Secretions. 
Assheton, R. 1916. Growth in Length. 
Hcrtwig, O. (ed.) 1906. Handbuch, etc., I, Chaps. 6-8. 
Jenkinson, J. W. 1913. Vertebrate Embryology, Chaps. 8 and 9. 
Kerr, J. G. 1919. Textbook of Embryology, II, Chaps. 7 and 8. 
Lillie, F. R. 1916. The Development of the Chick, 2nd Ed., Chap. 7. 
/Marshall, F. H. A. 1922. The Physiology of Reproduction, 2nd Ed. 
Meisenhcimer, J. 1921-4930. Geschlecht und Geschlcchter irn Tierreiche. 
Ncedham, J. 1931. Chemical Embryology, III, Sections 20-22, 24, and Epilegomena. 


Recent progress in vertebrate embryology has resulted so 
largely from the application of the experimental method that even 
the beginning student must acquaint himself with some of the 
methods used and the results so far obtained. Within the limits 
of this text only a few of the important fields in which the experi- 
mental method has been employed can be mentioned, and the 
student must be referred to more extended treatises for further 
information concerning this relatively new and important branch 
of embryology. 

The amphioxus and the frog have long been used by experi- 
mental embryologists, and more recently successful methods have 
been devised for the experimental study of the developing egg of 
the hen. Triton, in Germany, and Ambystoma, in this country, 
are urodele amphibia whose eggs have been particularly favorable 
for experimental embryology. The eggs of mammals, difficult to 
obtain, and, so far, impossible to orient during the early stages of 
embryology, have been employed to a lesser extent. 

The experimental embryologist alters the conditions under 
which the egg develops in the hope of determining the factors in- 
volved in particular developmental processes. It is appropriate 
that we conclude our study of early embryology with a short 
account of some of the experiments which bear directly on the 
organization of the fertilized egg, on differentiation during cleav- 
age and the formation of the germ layers, and on the direct 
effects of environmental factors upon development. 


The fertilized egg, as we have seen, is the product resulting 
from the union of two germ cells, the egg and the sperm. It 
contains two pronuclei, of maternal and paternal origin, re- 
spectively, as well as a mass of cytoplasm which is almost ex- 
clusively maternal in origin. The nuclei contain the parental 



contributions of genes, the units which together determine the 
hereditary characters of the developing individual. How the 
genes produce their effects is not known, but it is certain that 
they must act directly upon the cytoplasm. Accordingly we may 
turn first to experiments dealing with the nuclei of the fertilized 
egg, and second, to those concerned with the organization of the 
cytosome itself. 


The fact that the fertilized egg has the diploid number of 
chromosomes and of genes, while the two gametes have the 
haploid number, naturally leads to the question whether the 
diploid number is necessary to continued development. A con- 
siderable number of experiments bear directly upon this question. 

Artificial parthenogenesis. The frog's egg can be induced to 
develop by puncture with a finely pointed glass needle (Loeb and 
others). These artificially parthenogenetic eggs have given rise 
to tadpoles and frogs. Apparently the number of chromosomes is 
redoubled (diploid number), perhaps by a division of the chromo- 
somes without a corresponding division of the cell. But the 
genes are exclusively maternal in origin. 

Irradiated sperm. Sperms of the amphibian Triton, treated 
to an appropriate dosage of radium emanations, have their nuclei 
injured in such a way that they are unable to form normal pro- 
nuclei (Hertwig). But they retain their mobility and are able 
to penetrate the egg and induce development. The sperm head 
remains in the cytoplasm and passes to one or another of the 
developing blastomeres but takes no part in mitosis and ultimately 
degenerates. The number of chromosomes in the larval cells is 
usually haploid, although redoubling may occur. 

Irradiated eggs. Eggs of Triton have also been irradiated to 
kill the egg nucleus and then fertilized with normal sperms. These 
eggs develop with the haploid number of chromosomes, showing 
that either pronucleus, maternal or paternal, is adequate for 

Fertilization of enucleate eggs. In some marine inverte- 
brates, e.g., the sea urchin, the egg can be broken into fragments 
by shaking. Naturally only one fragment will contain the 
nucleus, but the enucleate fragments can be fertilized and will 



give rise to dwarf but otherwise normal larvae. This phenomenon 
is known as merogony. A similar result can be obtained in 
telolecithal vertebrate eggs such as those of Triton, where several 
sperms normally enter the egg. After the entrance of the sperm 

Egg nucleus 

FIG. 101. The experimental production of haploid larvae in Triton. A, fertilized 
egg with two sperm nuclei. B, same after constriction separating part of egg with 
diploid nucleus (right) from part with haploid nucleus formed by supernumerary 
sperm (left). C, showing relatively more advanced diploid embryo (right) and 
less advanced haploid embryo (left). D, diploid larva. E, haploid larva. (After 

it is possible to constrict the egg into two halves, by means of a 
fine hair loop, in such a way that the female pronucleus lies in one 
half (Fig. 101). This half will eventually have the diploid num- 
ber of chromosomes, for a sperm pronucleus will conjugate with 


the egg pronucleus. The other half will have only the haploid 
number. Both halves will develop into larvae, one of which will 
have haploid and the other diploid nuclei. 

Species hybrids. Many experiments have been made in the 
attempt to fertilize the egg of one species with a sperm from 
another species. Often as in the teleost fish 
(Moenkhaus), both pronuclei take part in the 
subsequent cleavage, although frequently the 
chromosomes from the two pronuclei (Fig. 102) 
form separate groups on the mitotic spindle 
(gonomery). But in other cases Hertwig has 
shown that the male pronucleus takes no part in 
subsequent cleavages, so that the embryo really 
develops parthenogenetically. 

Natural interspecific hybrids in both plant and 
animal kingdom are more common than for- 
F ' 102 -Ch Jnerly believed. Usually these interspecific hy- 
mosornes in ana- brids arc infertile, as the mule and many types 
phase of first of hybrid bony fish, but they often grow to larger 
cleavage of a hy- g j an( j are more ac tive (hybrid vigor) than the 

brid fish, Memdia ^ J ' 

egg and Fundulus parents. 

sperm, iiiustrat- The equivalence of the pronuclei. Although, 
mg gonomery. ag we j iave seen i n Chapter IV, the pronuclei may 

(After Mocnk- v , , t . , , . ,. . , , 

haus ) differ from one another in regard to individual 

genes, the experiments mentioned above indi- 
cate that a single set of genes, paternal or maternal, is adequate 
for the development of an egg. It must be recognized that the 
experimental haploid animals are frequently less vigorous than 
normal diploid forms. 


Polarity. The primary expression of the egg's organization 
is the polarity already impressed upon it in the ovary (page 37). 
That this polarity is itself not due to gravity is shown by the fact 
that frog eggs which are kept in motion during early development 
give rise to normal embryos (Morgan, Kathariner). But polarity 
is not immutable, for many experiments in which the eggs of frogs 
have been made to develop in an inverted position (Born, Pfliiger, 
Morgan) show that the yolk streams down through the egg, and 


cleavage begins in the relatively yolk-free region which was 
formerly the vegetal pole. 

Gradient. There seems to be good reason to suppose that the 
polar axis represents a metabolic axial gradient (Child), for when 
dilute solutions of lethal chemicals, e.g., potassium cyanide, are 
applied to the frog's egg (Bellamy), disintegration begins at the 
animal pole and continues toward the vegetal pole, which is the 
last part of the egg to be affected. 

Cytoplasmic materials. In some animals there seems to be a 
definite stratification of materials in the egg along the polar axis, 
but when this stratification is disturbed by whirling the eggs 
about in a centrifuge, the eggs develop with the original polar- 
ity undisturbed. On the other hand, in telolecithal eggs like 
that of the frog, centrifuging distorts the cytoplasmic framework 

Bilaterality. The animal pole marks the anterior end of the 
developing amphibian embryo. Its dorsal side is marked by the 
gray crescent which appears on the side opposite the point of 
entry of the sperm. Many observations (Jenkinson and others) 
show that the point of entry marks a second dorso-ventral axis 
and establishes the bilaterality of the developing embryo. But 
in parthenogenetic eggs (when development is initiated by punc- 
ture) the point of entrance of the needle seems to have no constant 
relation to the subsequent bilaterality of the egg. This would 
indicate (Huxley and de Beer) that the egg has an underlying 
bilaterality of its own which is not strong enough to withstand 
the stronger stimulus afforded by the entrance of the sperm but 
is apparent in parthenogenesis. 

Bellamy has described a second axial gradient in the frog's egg 
shown by the action of potassium cyanide in which the high point 
centers in the gray crescent. This is the dorso-ventral axis of 
the embryo, which is therefore normally determined by the 
entrance point of the sperm. 

Asymmetry. The vertebrate embryo is not, strictly speaking, 
bilaterally symmetrical. A third axis or gradient from one side 
to the other (usually left to right) is often apparent, as seen in the 
development of the atriopore on the left side of the tadpole, the 
fact that the heart of the chick develops on the right side, and the 
fact that the head turns to the right in torsion. The stomach in 


all vertebrates is twisted to the left of the mid-line, and many 
other examples might be mentioned. When this asymmetry is 
reversed we have the phenomenon known as situs inversus, and 
this condition can be reproduced experimentally by developing 
the egg in a lateral temperature gradient and in other ways. 
Thus the egg of the hen when overheated on the left side develops 
situs inversus. It has been shown by Spemann that, when two 
blastomeres which would ordinarily produce the right and left 
sides of an embryo are separated by a hair loop, the left-hand 
blastomere gives rise to a normally asymmetrical embryo, while 
the right-hand blastomere gives rise either to an embryo with 
normal asymmetry or to one with situs inversus. 

These few examples of experiments on the fertilized egg indicate 
that the egg is a complex system with a definite organization 
indicated by its three axial gradients corresponding to its three 
spatial dimensions, viz., an antero-posterior gradient (polarity), 
a dorso- ventral gradient, and frequently a left-right gradient. 
Furthermore, the system contains two complete sets of chromo- 
somes and genes, either one of which is adequate in further 


Cell-lineage studies seemed to indicate that the dividing egg is 
becoming a mosaic of blastomeres, each set apart from the others 
to form a specific portion of the embryo. Roux (1888) was the 
first to realize that this might be tested experimentally. He 
destroyed one of the J-blastomeres of the frog's egg and observed 
that the other gave rise to a J-embryo, which later regenerated 
the missing portion. 

Later investigators devised a number of methods by which 
blastomeres could be separated from each other, by shaking them, 
cutting them apart with fine needles, constricting them with fine 
threads, or placing them in artificial calcium-free sea water. 
Blastomeres of marine eggs in this medium separate immediately, 
and when returned to normal sea water continue their develop- 
ment without further separation (Herbst). 

Regulation and mosaic eggs. The results of their experi- 
mentation seemed to indicate that in some eggs, e.g., those of the 
amphioxus (Fig. 103), either of the |-blastomeres might, when 



separated, give rise to complete embryos (Wilson). These were 
called regulation eggs and were said to have indeterminate cleav- 
age. In others, such as Styela (Conklin) or the mollusc Dentalium 
(Wilson), the i-blastomeres give rise only to J-embryos (Fig, 103). 
These were called mosaic eggs and were said to have determinate 

Experiments on frog's eggs had been inconclusive until recently 
an improved technique has made it possible to separate blasto- 
meres of the two-cell stage completely (Schmidt, 1930, 1933). 

FIG. 103. Diagram to show the fate of isolated blastomeres from mosaic and regu- 
lation eggs. A, mosaic egg of Dentalium. At left, a complete embryo produced 
by entire egg; at right, partial embryos produced by the {-blastomeres when 
artificially separated- B, regulation egg of Amphioxus. At left, embryo pro- 
duced by entire egg; at right, perfect dwarf embryos produced by i-blastomeres. 
(After Wilson.) 

These experiments show that each of the t-blastomeres can give 
rise to a complete and perfect larva, provided only it contains 
some of the gray crescent region. If, on the other hand, the egg 
is so constricted that the first cleavage divides it into an animal 
and a vegetal half, the animal half, containing the gray crescent, 

FIG. 104. Embryos arising from separated J-blastomeres of the newt's egg. A, the 
constriction separates the dorsal and ventral halves of the embryo. B, the con- 
striction separates the right and left halves. C, perfect embryo arising from the 
dorsal |-blastomere. D, mass of cells arising from ventral i-blastomere. E, two 
perfect embryos arising from right and left ^-blastomeres respectively. (After 
Spemann.) U60) 



gives rise to a complete embryo, while the vegetal half, lacking 
this region, is unable so to organize itself (Fig. 104). The im- 
portance of the gray crescent as the seat of the organizer is dis- 
cussed on page 169. This seems to indicate that Roux's results 
were due to the presence of the injured blastomere inhibiting 
complete development on the part of the uninjured blastomere. 
In this connection it is interesting to note that Witschi (1927) has 

FIG. 105. Experiment demonstrating equality of nuclei formed during cleavage 
(Triton). A ligature has been tied around the fertilized egg restricting the nucleus 
to the right-hand portion. A, l(>-cell stage, one nucleus passing into left-hand 
portion. B, ligature tightened to separate the two portions. C, perfect embryos 
formed by the separate portions. The nucleus of a A th-blastomere equivalent to 
that of a complete zygote. (After Spemann.) 

described a case in which two eggs were found in a single chorion. 
Each of them was flattened on the side next to its neighbor and in 
later development showed deficiencies in the corresponding region. 
A beautiful demonstration that it is the cytoplasm and not the 
nucleus which is concerned with differentiation during cleavage 
is afforded by an instructive experiment of Spemann. If the egg 


is tied off before cleavage so that the nucleus is confined in one of 
its halves (Fig. 105), all cleavage planes will be restricted to that 
half until eventually a cleavage plane, in this case at the fourth 
cleavage, coincides with the plane of constriction. The nucleus 
which enters the previously enucleate half is naturally one which 
would serve a ^-blastomere. If the loop is now tightened until 
the two haves are completely separated, the portion containing 
this single nucleus will give rise to an embryo like the one from 
the portion containing the fifteen nuclei and exactly like one 
arising from a complete fertilized egg. 

Pressure experiments. Further examples of the regulative 
power of some eggs may be seen in pressure experiments. If the 
eggs of the frog are placed between glass plates during cleavage, 
the third cleavage planes will be meridional instead of latitudinal, 
and the fourth cleavage plane is latitudinal (Fig. 106). Now if 


B D 

FIG. 106. Diagram to show new relationship of blastomeres in frog's egg resulting 
from pressure during cleavage. A, normal 8-cell stage. B, 8-cell stage formed 
under pressure. C, normal 16-cell stage. D, 16-cell stage formed under pressure. 
Cells normally in animal hemisphere shown in stipple. (Suggested by a diagram 
in Wells, Huxley and Wells.) 

the eggs are released, their later development will be quite normal 
even though the blastomeres are occupying positions unlike those 
which they hold ordinarily. 

Double embryos. Still another example may be seen in the 
eggs of Triton. If these are freed from the egg envelopes, the 
blastomeres at the two-cell stage assume a dumb-bell shape. 
Mangold discovered that, by placing one embryo in the two-cell 
stage over another (Fig. 107), a double embryo resulted almost 
exactly similar to a single embryo in the four-cell stage, and would 



develop as such, provided only that the gray-crescent regions of 
the two fell in the same plane. Otherwise double monsters re- 
sulted. We shall see the importance of the gray-crescent region 
more clearly in a later section dealing 
with the organizer which develops in 
this region. 

Chemo-differentiation. It is 
quite clear from these experiments 
that the developing egg of the regu- 
lation type possesses a very great 
plasticity in the early stages of de- 
velopment as compared to the mosaic 
type illustrated by the egg of the 
tunicates. It may be assumed that 
the difference between these two 
types lies in the time at which defi- 
nite organ-forming substances are 
segregated in the cytoplasm of the 
egg. Conklin has demonstrated 
that these regions are segregated 
after fertilization in the egg of the 
tunicate, whereas in amphibian eggs 
the only segregated region is that of the gray crescent. Huxley 
(1924) has suggested the term chemo-differentiation for the segre- 
gation of organ-forming substances. A good example is seen in 
the first division of the egg of Dentalium, the mollusc referred to 
above where a polar lobe passes completely to one or the other of 
the first -|- blastomeres. The cell receiving this lobe gives rise to 
the apical organ, mesoderm, foot, and shell. Here the very first 
division of the fertilized egg is determinate and dependent upon 
the segregation of the organ-forming substance found in the polar 
lobe (Fig. 108). 

Monovular twins and monsters. The extreme plasticity of 
the vertebrate egg as seen by the fact that either two separate 
individuals or duplicate monsters may be formed from the com- 
plete or partial separation of blastomeres suggests an explanation 
of identical twins and the duplicate monsters which play so large 
a part in the study of teratology. It is generally accepted that 
identical, as distinguished from fraternal, twins are the product 

FIG. 107. Double embryo arising 
from fusion of 2-cell stages of 
Triton alpestris (above and below) 
and Triton taeniatus (right and 
left) when laid over each other 
crosswise. Note that a new 
cleavage is under way in all 
blastomeres. (After Mangold and 



of a single fertilized egg which has divided completely during early 
embryology, whereas the duplicate monsters, ranging from Sia- 
mese twins to monsters in which one individual is but a parasite 
upon the body of the other, result from incomplete separation. 
These identical twins are always of the same sex. Ordinary or 

FIG. 108. Diagram to show possible distribution of organ-forming substances in 
mosaic and regulation eggs. 'A, immature egg. B, mature egg showing stratified 
organ-forming substances. C, cleavage with equal division of chromosomes. 
D, segregation of one organ-forming substance in left-hand J-blastomere. E, 
equal division of organ-forming substances between the |-blastomeres. (After 

fraternal twins (triplets, etc.) are supposed to be the product of 
separate eggs which ovulated and were fertilized at about the 
same time. Such twins are frequently of different sexes. In this 
connection we might mention the free-martin, a sterile female 
twinned with a male, not infrequent among cattle, and supposed 
to result from one of two eggs which develop a common chorion 
and therefore a common blood stream. It is supposed that a male 


hormone circulating in the common blood stream inhibits the 
normal development of the female twin, so resulting in the 
production of the sterile free-martin (Lillie). 


The amphibian embryo is remarkably hardy and during the 
early stages of development will endure very severe operations. 
The work of Harrison in this country and of Spemann in Germany 
has resulted in the perfection of a method of removing portions 
of an embryo (micro-dissection) and grafting them into a new 
environment, where they will continue development. The em- 
bryo from which the portion is removed is known as the donor; 
the removed portion is called the graft (transplant) ; and, when 
the portion removed is transplanted into another embryo, the 
second embryo is termed the host. 

The accompanying diagram (Fig. 109) will bring out some of 
the methods which have been developed in transplantation experi- 
ments. Thus the graft may be transplanted into another portion 
of the same embryo (homoplastic transplantation). 1 It may be 
transplanted into another embryo of the same species (hetero- 
plastic transplantation). It may even be transplanted into an 
embryo of another species or genus (xenoplastic transplantation). 

Another method which has brought interesting results is to 
transplant the removed portion into a nutrient medium and allow 
it to develop there under sterile conditions (explantation). This 
is also known as cultivation " in vitro/ ' which means in glass. 
Another ingenious technique is to transplant the graft into a 
cavity of another embryo and allow it to develop there. The 
example shown in the diagram is of a bit of embryonic tissue 
transplanted into the eyeball of a tadpole, which acts as a nutrient 
chamber. Hoadley and others have developed a technique of 
grafting chick-embryo tissue from a donor to the chorio-allantois 
of a host. Such a technique is called mterplantation (implan- 

Plasticity (dependent differentiation). In the amphibian egg, 
which is of the regulation type, it has been demonstrated that the 

1 Some' investigators use autoplastic = homoplastic; homoplastic = he teroplastic; 
and heteroplastic = xenoplastic. 



presumptive organ regions of the blastula, (and until about the 
middle of gastrulation) are quite plastic, i.e., can be transplanted 
into other localities and will give rise to the organs appropriate 

' Graft 






FIG. 109. Diagrams to show different methods of transplantation, etc. A, homo- 
plastic transplantation. B, heteroplastic transplantation (both donor and host 
of same species). C, xenoplastic transplantation (donor and host of different 
species). D, explanation (in vitro). E, interplantation. (Based on a diagram of 

to the new locality. Thus, material which is presumptive epi- 
dermis can be transplanted into a region where it will become 
neural plate, mesoderm, or even endoderm. Or on the other hand, 
material which is presumptive endoderm can be made to develop 
into ectoderm or mesoderm by transplantation. The only ex- 
ception to this rule is the region where the dorsal lip is to form. 



This will never give rise to anything except dorsal lip and the 
structures arising from the dorsal lip. This exception will receive 
special attention later (page 169). 

Very instructive experiments are those in which material is 
transferred from a species with heavy pigmentation (Triton 
taeniatus) to one with light pigmentation (Triton cristatus). 

i$^^>*4&f* f tf& 

FIQ. 110. Xenoplastic transplantation between Triton taeniatus (dark), the donor, 
and Triton cristalus (light), the host to show early plasticity. A, immediately 
after transplantation. B, the transplant developing in the gill region. C, the 
gills of the transplant relatively more advanced. D, section through C in the gill 
region. (After Spemann.) 

Here the graft preserves its racial character of pigmentation while 
otherwise conforming to the development of the host. Figure 110 
illustrates such an example of xenoplastic transplantation. The 
light-colored graft from T. cristatus has developed into part of the 
neural tube of the host, where it stands out by reason of its light 
color. In the reciprocal transplantation (Fig. Ill), the dark 
graft from T. taeniatus has given rise to the right external gills of 
the host. 



The loss of plasticity (self-differentiation). After gastrulation 
is well under way this plasticity seen in earlier stages is lost. The 
various regions of the embryo have become determined and 


A B C 

FIG. 111. Reciprocal transplant to that shown in Fig. 110. Here T. cristatus 
(light) is the donor and T. taeniatm (dark) is the host. A, after transplantation. 
B, the transplant developing in the neural plate (region of the brain). C, section 
in later stage showing transplant developing in forebrain. (After Spemann.) 

thenceforth will give rise only to the structures normally develop- 
ing from them. In other words, the amphibian embryo does not 
undergo chemo-differentiation until this time. From now on it 
is a real mosaic. Figure 112 shows a neurula in which the various 

Ear field 

Eye field 

Neural tube field 


Nose field 
Lens field 

Balancer field 




Forelimb field 

Hindlimb field 

FIQ. 112. Diagram of an amphibia neurula showing organ fields as determined by 
transplantation experiments. (After Huxley and de Beer.) 

organ fields are determined. If a bit of tissue is removed from 
the eye-field and transferred to the flank of another neurula 
(Fig. 113), it will give rise only to an eye, even in its new and 
abnormal environment. 

Similar experiments have been carried on with the chick (im- 
plantation on chorio-allantois), and it has been proved that the 



eye-field, ear-field, limb-buds, and other regions will develop and 
give rise only to the respective organs. 

Very striking results have been obtained by implanting portions 
of rat embryos on the chorio-allantois of the chick, and a con- 
siderable amount of self-differentiation has been demonstrated. 



FIG. 113. Self-differentiation in the toad Bombinator. A, donor in early neurula 
stage showing region from which graft was taken. B, host in late neurula stage. 
C, section through later embryo of host, showing graft forming optic cup in region 
normally occupied by pronephros. (After Spemann.) 

The organizer. The loss of its early plasticity by the embryo 
seems to be due to the presence of an organizer (organisator) as 
discovered by Spemann. In the amphibian embryo this is the 
dorsal-lip region, already mentioned. It will be recalled that this 
region alone of the presumptive organ fields of early gastrulation 
did not show the phenomenon of plasticity. Wherever it is 
transplanted it will become dorsal lip. But the most striking 
thing about this dorsal-lip region is that wherever it is transplanted 
it will bring about involution, and will transform the surrounding 
material into organ fields such as are ordinarily found about the 
dorsal lip. In a word, the grafted dorsal lip organizes a new, 



secondary, embryo about itself, quite independent of the embryo 
which is organized about the dorsal lip of the host (Fig. 114). 

The organizer itself undergoes involution beneath the surface 
of the host and becomes the notochord and the somite-mesoderm 
of the secondary embryo. The other structures, such as neural 

Primary neural 


Primary gut 

Secondary pronephric duct 
Secondary gut 





FIG. 114. Effect of transplanting organizer. A, dorsal view of host (Triton 
taeniatus) in neurula stage. B, right side view at same stage showing secondary 
neural plate induced by organizer (dorsal lip region) of the donor (Triton cristatus) 
shown in white. C, later stage showing primary embryo in side view arid second- 
ary embryo in dorsal view. D, transverse section through C. (After Spemann 
and Mangold.) 

plate, eyes, ears, kidney, heart, etc., arise from host tissue which 
has been brought under the influence of the organizer. Even 
after gastrulation this influence is continued, as can be shown by 
the following experiment. A bit of gastrocoel roof (notochord 
and mesoderm, in the urodeles), when transplanted into the 


side of the gastrocoel, will induce the formation of neural folds 
above it. 

So great are the powers of induction possessed by the organizer 
that it can cause presumptive ectoderm to become mesoderm or 
endoderm, and conversely, presumptive mesoderm can be trans- 
formed into ectoderm. 

It is noteworthy that the organizer can exert its influence even 
in xenoplastic transplantation, e.g., the organizer from a toad can 
induce the formation of a secondary embryo in .a newt. Appar- 
ently the effects of the organizer are physico-chemical in nature, 
for the dorsal-lip region can be narcotized, boiled, or even dried, 
and still induce the formation of a secondary embryo. It is 
suggestive that bits of agar after being in contact for some time 
with an organizer are themselves capable of producing induction. 
There is reason to believe that the glycogen (animal starch) 
content of the organizer has something to do with its effects, and 
quite recently, it has been reported that cephalin will bring out 
about the induction of a secondary embryo. Many parts of the 
adult vertebrate are capable of bringing about induction, but in 
the living embryo, the chemical substance responsible is found 
only in the organizer itself. 


Many experiments have been carried on in the attempt to find 
the definite results produced on the developing embryo by 
changes in the environment. These investigations have estab- 
lished normal limits of temperature, etc., within which develop- 
ment can be completed. Within these limits, although develop- 
ment may be altered as to rate, etc., it is nevertheless carried on 
to a successful outcome. Beyond these limits the alterations are 
so profound as to produce monsters or cause death. Among the 
factors susceptible to experimental control are gravity, heat, 
light, the chemical constitution of the environment, and food. 

Gravity (and centrifugal force). It has been remarked (page 
156) that the original polarity of the egg is not due to any effect 
of gravity. In telolecithal eggs, however, gravity may have some 
effect on the course of development. Thus frog's eggs when force- 
fully inverted may give rise to duplicate monsters. The hen's egg 
if not rotated at regular intervals fails to hatch. It has been 


shown (Dares te, 1877) that this is due to the failure of the yolk 
sac to complete its development. It adheres to the allantois and 
cannot be retracted into the body as in normal development. 

The influence of gravity may be shown in an exaggerated 
manner by prolonged centrifuging. It was found by 0. Hertwig 
that, if the frog's egg is centrifuged during cleavage, the yolk is so 

concentrated in the vegetal hemi- 
sphere that the cleavage planes fail to 
cut through it and the end result is 
meroblastic cleavage suggestive of 
that seen in the chick (Fig. 115). 

Heat. The rate of development 
is directly affected by temperature. 
Thus for the egg of the frog (Rana 
fusca, Hertwig) the normal tempera- 
FIQ. 115. Vertical section ture is about 15-16 C. From this 

through blastula of a frog's egg ^ to about 2 0-22 C., devel- 

following centrifuging. (After / t . n i i 

Hertwig.) opment continues normally; beyond 

this limit it is abnormal, death ensuing 

rapidly at 30 C. Below 15 C., development is retarded pro- 
gressively with the drop in temperature, and at C. cleavage 
ceases completely. 

For the hen's egg, Kaestner determined the optimum tempera- 
ture for normal development to be between 35 and 39 C. 
(95-102 F.). The maximum temperature tolerated is 43 C., 
the minimum 28 C. (20-21 C., Edwards). 

Eggs of either frog or hen which have been exposed to extreme 
heat or cold and then returned to the optimum temperature often 
develop abnormally. A common type of monster is one in which 
neural plate and notochord are split (spina bifida). 

Very striking results have been obtained by subjecting the eggs 
of the frog or the hen to a temperature gradient, that is, control- 
ling the temperature so that one side is hotter or colder than the 
other. If the gradient runs along the polar axis, and the greater 
heat is applied to the animal pole, the result is that the embryos 
and larvae have overlarge heads; if the higher temperature is ap- 
plied to the vegetal pole, the head region is subnormal. When the 
temperature gradient is applied laterally, the development of the 
heated side proceeds more rapidly than that of the cooled side. 


It may be concluded that, within the limits of toleration, de- 
velopment is accelerated by increased temperatures and retarded 
by decreased temperatures. 

Light and other forms of radiation. In spite of a considerable 
number of experiments designed to determine the effects of 
definite intensities and wavelengths of light upon the developing 
embryo, the results are as yet too inchoate to be discussed in an 
elementary text. 

Ultra-violet light, X-rays, and radium emanations in extreme 
dosage cause the cessation of development. In smaller dosage, 
they bring about anomalies (abnormalities in structure caused by 
disturbances in development). It should be remembered that the 
work of Miiller and others indicates that these agents accelerate 
the rate of mutation of Drosophila genes, and so induce genetic 
point mutations as well as developmental anomalies. 

Chemical composition. The chemical composition of the sur- 
rounding medium affects profoundly the nature of development. 
The embryo cannot develop without oxygen, for it cannot live 
without respiration. It has been pointed out by Morgan that 
frog's eggs in the very center of the egg mass often develop ab- 
normally (spina bifida, etc.). And it has long been known that 
the hen's egg ceases development if the pores of the shell are 
closed by water glass, varnish, or other agents. 

Water, too, is an essential. The growth of the embryo de- 
pends upon the absorption of water, and all embryos must undergo 
their development within a watery medium. Even the terrestrial 
embryo has its private pond in the amnion. A slowing up in the 
rate of development, accompanied by abnormalities and a large 
percentage of deaths, results from incubating hen's eggs in a 
desiccator. The percentage of water in the frog's egg increases 
steadily during the first two weeks of development. 

A very striking series of experiments was carried on by Herbst 
on the development of the sea urchin in artificial sea waters which 
had been made up omitting one after another of the elements 
found in normal sea water. Jenkinson, summarizing the evidence 

" The experiments which we have been considering are unique 
of their kind, and it is impossible to exaggerate their importance. 
For, whatever may be the ultimate explanation of the facts, there 


can be no doubt whatever that the most complete demonstration 
has been given of the absolute necessity of many of the elements 
occurring in ordinary sea water, its normal environment, for the 
proper growth and differentiation of the larva of the sea urchin. 
Nor is this all. Some of the substances are necessary for one 
part or phase of development, some for another, some from the 
very beginning, others only later on. Thus potassium, magne- 
sium, and a certain degree of alkalinity are essential for fertiliza- 
tion, chlorine and sodium for segmentation, calcium for the 
adequate cohesion of the blastomeres, potassium, calcium and 
the hydroxyl ion for securing the internal osmotic pressure 
necessary for growth, while without the sulph-ion and magnesium 
the due differentiation of the alimentary tract and the proper 
formation of the skeleton cannot occur; the secretion of pigment 
depends on the presence of some sulphate and alkalinity, the 
skeleton requires calcium carbonate, cilia will only beat in an 
alkaline medium containing potassium and magnesium, and 
muscles will only contract when potassium and calcium are 

The addition of chemicals to the medium has resulted in many 
interesting disturbances in development. We can call attention 
here to two only. In the sea urchin it was found that the addi- 
tion of lithium salts to sea water caused the embryo to undergo 
a very curious form of gastrulation, in which the endoderm and 
mesoderm were evaginated instead of being invaginated (Herbst). 
Such an embryo is called an exogastrula. 

Quite recently, Holtfreter (1933) has induced exogastrulation 
in the egg of Triton by removing the egg envelopes and placing 
the developing egg in weak Ringer's salt solution. In the cases 
where development continued for some length of time (Fig. 116), 
it was discovered that the embryo developed in two parts, an 
ectodermal portion with no differentiation, connected by a narrow 
isthmus to a mesendodermal portion in which differentiation 
proceeded, but in an abnormal fashion. The embryo is inside-out. 
The mesendodermal portion of the exo-embryo develops a typical 
notochord, somites, kidney, gonad, a heart (empty), and a diges- 
tive tube, in which all the typical regions are indicated, including 
visceral pouches. These results confirm those of transplantation 
and explanation experiments discussed in an earlier section. 



Food (including hormones and vitamins). The amount 
and kind of food supplied to the developing young naturally 
affect the subsequent development. Thus, if frog tadpoles are 
fed on an exclusively vegetarian diet, the intestine becomes much 
longer than when an exclusively meat diet is offered. Specific 
foods often result in equally definite changes in the body. Thus 

Epidermis v 

Kidney tubule 
Trunk muscles 

Head muscles 
Mouth endoderm 


FIG. 116. Exogastrulation in Ambystoma. A, B, exogastrulae showing direction 
of displacements during exogastrulation, compare Fig. 74. C, section of later 
exo-ernbryo. (After Holtfreter in Huxley and de Beer.) 

Gudernatsch discovered that frog tadpoles fed on thyroid tissue 
grew less rapidly but underwent metamorphosis much more 
rapidly than the controls. Thymus-fed tadpoles, on the other 
hand, had a retarded metamorphosis accompanied by excessive 
growth. Later investigations indicate that the effects of thyroid 
are due to a hormone formed by this gland (thyroxin), which is a 
definite factor in bringing about amphibian metamorphosis. 

It is interesting to note that by the use of thyroid or thyroxin 
the Mexican axolotl (Fig. 117) may be induced to undergo 
metamorphosis, when it becomes a normal Ambystoma tigrinum. 
Otherwise the axolotl becomes sexually mature in the larval 
condition (neoteny), and was, therefore, long thought to be a 
separate species. 



In this connection we may refer briefly to the many experi- 
ments dealing with the effects of the various endocrine glands 
when given as food or as transplants and the effects produced 
when these glands are removed at their first appearance (extirpa- 
tion). Without going into details, for the results of these experi- 
ments are sometimes ambiguous, we may say only that they 

FIG. 117. Metamorphosis in Ambystoma. A, neotenic larva (axolotl). B, meta- 
morphosed adult. (After Diirken.) 

indicate the importance of hormones in embryonic as well as in 
adult life. 

The role of the vitamins in the metabolism of the embryo is too 
little understood at the present time for us to do more than allude 
to this subject. Vitamin E is often called the anti-sterility 
vitamin because its absence from the diet results in loss of the 
reproductive power. Adamstone (1931) in this laboratory has 
shown that the chick embryo produced by hens on a vitamin-E- 
free diet dies early in development following extensive disturb- 
ances in the blood-vascular system. 



Experimental embryology demonstrates that development is 
epigenetic. Given a suitable inheritance of genes, and a favorable 
environment, development proceeds normally through stages of 
increasing complexity. Any alteration, either in the genetic 
complex or in the factors of the environment, will bring about 
alterations in development. 

The fertilized egg shows a definite organization as seen in its 
polarity and symmetry. These seem to be the expression of 
axial gradients. Sooner or later the cytoplasm of the egg under- 
goes chemo-differentiation and develops organ-forming substances 
sooner in mosaic eggs, later in regulation eggs. 

Cleavage segregates the organ-forming substances as they are 
differentiated, with the result that the isolated blastomeres of 
mosaic eggs have a limited potency, those of regulation eggs have 
a greater potency. 

During germ-layer formation, the presumptive organ regions 
are segregated into the different germ layers. Among the verte- 
brates this reorganization is effected by an organizer, which in 
the frog is associated with the dorsal lip of the blastopore, and in 
the chick with the homologous primitive streak. 

Even in regulation eggs a mosaic stage is established during 
germ-layer formation. The different organ fields are now deter- 
mined, the earlier plasticity disappears, and each field is capable 
only of self-differentiation. 


Allen, E. (ed.) 1932. Sex and Internal Secretion. 

Bertalanffy, L. von, and Woodger, J. H. 1933. Modern Theories of Development. 

Brachet, A. 1931. L'oeuf et les factors de 1'ontoge'nese. 

Brambell, F. W. R. 1930. The Development of Sex in Vertebrates. 

de Beer, G. R. 1926. Introduction to Experimental Embryology. 

Child, C. M. 1915. Individuality in Organisms. 

Duesberg, J. 1926. L'oeuf et ses localisations germinales. 

Diirken, B. 1932. Experimental Analysis of Development (trans). 

Faure-Frerniet, M. E. 1925. La cine'tique du deVeloppement. 

Huxley, J. S., and de Beer, G. R. 1934. The Elements of Experimental Embryology. 

Jenkinson, J. W. 1909. Experimental Embryology. 

1917. Three Lectures on Experimental Embryology. 

Korschelt, E. 1927-1931. Regeneration and Transplantation. 
Morgan, T. H. 1928. Experimental Embryology. 
1934. Embryology and Genetics. 


Needham, J. 1932. Chemical Embryology. 
,/fWman, H. H. 1923. The Physiology of Twinning. 
^Russell, E. S. 1930. The Interpretation of Development and Heredity. 

Schleip, W. 1929. Die Determination der Primitiventwicklung. 

Weiss, J. 1930. Entwicklungsphysiologie der Tiere. 

Wilson, E. B. 1925. The Cell in Development and Heredity, 3rd Ed. 




The tissues derived directly from the endoderm are for the 
most part of the epithelial type and form the inner lining of the 
gastrocoel and the organs that arise therefrom. These organs 
are grouped into two closely connected organ systems, the diges- 
tive system and the respiratory system. The digestive (enteric) 
tube, however, becomes ensheathed in splanchnic mesoderm which 
contributes largely to the ultimate structure of the organ systems 
just mentioned. Furthermore, this tube opens to the exterior 
at both the anterior and posterior ends by means of two ecto- 
dermal pits, the stomodeum and proctodeum, respectively. All 
three germ layers, therefore, contribute to the organogeny of these 

The stomodeum. There is an ectodermal invagination on 
the ventral side of the head to form the stomodeum (Fig. 118), 


Neural tube 







FIG. 118. Diagram of an early vertebrate embryo, to show endodermal derivatives. 

which is bounded on the sides by the maxillary ridges and on the 
rear by the mandibular ridges. The rupture of the oral plate, 
which separates the stomodeum from the fore-gut, results in the 
formation of the oral cavity, or mouth. From the stomodeum 
another invagination, the hypophysis, grows upward in front of 
the fore-gut, and eventually fuses with an evagination from the 
floor of the neural tube, the infundibulum, to form the pituitary 




gland, an organ of internal secretion. As the stomodeum joins 
the fore-gut a little posterior to the anterior end of the latter 
cavity, there is a blind pocket of endoderm, anterior to the 
mouth, called the preoral gut. 

The oral cavity. The cavity of the mouth is a compound 
structure, derived in part from the ectodermal stomodeum and 

of mouth t l't' ! '- l 'f t ~*'W. - 


Dentine - './i;f: M 

- -. * .- . .' 
''f- J '\ f ' : ~; ;' ' V '- ' \ y '?- 1 A ! i Dental ridge 

^\-. '-"" !-v.V'~/* v .^'.t 

> Enamel organ 

FIG. 119. Diagram to show origin of vertebrate tooth (lower jaw). 

in part from the endodermal fore-gut. The boundary line be- 
tween these is soon lost after the rupture of the oral plate owing 
to unequal local growth of the different regions of the mouth. 
The boundaries of the mouth are the upper jaws, formed from the 
maxillary ridges, and the lower jaws, derived from the mandibular 

Visceral pouches 

Dorsal pancreas 



' epithelial bodies 



Ventral pancreas 

Hepatic diverticulum 

FIG. 120. Diagram showing derivatives of vertebrate fore-gut. 

ridges. On these ridges the teeth arise in exactly the same way 
as the placoid scales of the elasmobranchs (page 230). Two ele- 
ments are concerned : an ectodermal enamel organ, shaped like an 
inverted cup; and a mesodermal dental papilla, which fills the 
cavity of the enamel organ. The enamel organ gives rise to the 
outer enamel layer of the tooth, while the papilla forms the dentine 
(Fig. 119). The dentine is in the general form of a hollow cone, 


the cavity of which is filled with connective tissue, nerves, and 
blood vessels. The tongue (Fig. 120) is also a compound organ, 
arising from an endodermal primary tongue which is formed from 
the floor of the pharynx in the region of the hyoid arches, and 
from an ectodermal secondary tongue which arises from the 
floor of the oral cavity in front of the thyroid gland (page 184). 
Into the tongue a migration of mesoderm takes place, by means 
of which the musculature is formed. The glands of the mouth 
(salivary glands, etc.) arise from the ectodermal lining of the 
mouth. The taste buds, however, are endodermal (Holtfreter, 
1933). The connection between the oral cavity and the nasal 
cavity will be discussed in Chapter X. 

The pharynx. The region of the fore-gut which follows the 
oral cavity is the pharynx, particularly important on account of 
the respiratory organs and other structures which arise from it. 

Respiratory organs. Respiratory exchange may take place 
in any thin epithelium in which the blood corpuscles are brought 
into contact with the oxygen-carrying medium. These epithelia 
may be either ectodermal or endodermal in origin. Thus, we 
find that among the amphibia, respiration may take place in the 
skin as a whole (lungless salamanders) ; in specialized outgrowths 
on the visceral arches, external gills ( Necturus) ; or in the so-called 
" hairs " of the African frog, Astylosternus. In this group are to 
be found also examples of endodermal respiratory organs, the in- 
ternal gills and lungs. Internal gills are otherwise found only 
among the fish, while the lungs are characteristic respiratory 
organs also of the amniotes. 

The internal gills. The internal gills (branchiae) arise in the 
visceral clefts (Fig. 120) common to all chordates. Among the 
aquatic vertebrates these are typically six in number (see Table 8, 
page 131). In the cartilage fish the first cleft (the spiracle) opens 
on the dorsal side of the head and is otherwise modified. The 
clefts are separated by the visceral arches, of which the first is 
known as the mandibular arch and the second is called the hyoid 
arch. The visceral clefts are formed by the coming together of 
paired evaginations of the endoderm (visceral pouches) and com- 
plementary invaginations of the ectoderm (visceral grooves). 
The ectoderm and endoderm come into direct connection to form 
closing plates. Later, these plates rupture and a series of finger- 


like projections grow out into the cleft from the anterior and 
posterior sides of each arch. These filamentous processes usually 
fuse to form a demibranch (Fig. 197). The demibranchs in some 
fish are apparently of endodermal origin, while in the amphibia 
they are derived from the ectoderm. It is interesting to note that 
in the spiracle of the cartilage fish a gill-like structure, the 
pseudobranch, develops. In amphibians and the amniotes gener- 
ally the first visceral pouch does not open to the exterior but 
gives rise to the tympanic cavity and auditory tube (see Chapter 
X). In all fish except the elasmobranchs, a projection grows 
back from the hyoid arch to cover the remaining visceral clefts. 
This is the operculum. Internal gills do not appear in the de- 
velopment of the amniotes ; but the visceral clefts, or at least the 
visceral pouches and grooves, are of invariable occurrence. 

The lungs. In all the vertebrates except the cyclostomes and 
cartilage fish, there develops from the pharynx a sac (or a pair 
of sacs) which becomes the air bladder in pisces and the lungs 
in tetrapoda. We shall confine our attention here to the develop- 
ment of the lungs (Fig. 120). The first indication of lung forma- 
tion is the appearance of a longitudinal groove in the floor of the 
pharynx posterior to the last pair of visceral pouches. This is 
the tracheal groove. This groove separates from the pharynx, 
the process commencing at the posterior end, so that the dorsal 
portion of the tube, or esophagus, is separated from the ventral 
portion, or trachea, except for a narrow opening, the glottis. 
The trachea grows backward rapidly arid divides into two lobes, 
the primordia of the lungs. There is some evidence that the 
trachea is bifurcated from its first appearance, suggesting that 
the lungs arise from paired primordia. In the birds and mammals 
the lung primordia subdivide many times to form the bronchi, or 
branches of the respiratory tree. 

The thyroid gland. This structure arises as a median ventral 
evagination of the pharyngeal floor between the primary and the 
secondary tongue primordia or at the level of the hyoid arches. 
The diverticulum grows downward and expands at its distal 
end (Fig. 120). Eventually, its connection with the pharyngeal 
floor, the thyroglossal duct, becomes occluded and disappears, 
and the gland itself subdivides into a mass of vesicles which 
migrate backward and assume somewhat different positions in 



various vertebrates, often ending as a paired organ on either side 
of the trachea. 

The epithelial bodies. In all the vertebrates there arise, 
from the upper or lower angles of the visceral pouches, small 
buds of epithelium which often give rise to endocrine glands of 
varying and mostly unknown function (Figs. 120, 121). 
The dorsal buds (except among the mammals, where conditions 
are reversed) contribute in varying number to the formation of a 
large gland, the thymus, which loses connection with the pharynx 
and moves backward to its definitive position, which differs 
according to the form studied. The remainder of the dorsal 





Fia. 121. Diagrams showing origin of epithelial bodies in A, frog; B, chick; 

and C, man. 

bodies become lymphoid and degenerate. The ventral buds 
(absent in fish) detach themselves from the pharyngeal wall 
and take up varying positions. Among the mammals it is 
the ventral buds which form the thymus, while the dorsal buds 
of the third and fourth pouches move to the sides of the thyroid 
gland where they are known as the parathyroids. 

The esophagus. The digestive canal behind the pharynx 
becomes specialized into four regions: (1) the esophagus; (2) 
the stomach; (3) the intestine and its derivatives; and (4) the 
cloaca. Of these, the esophagus (Fig. 120) remains comparatively 
unspecialized; it is a narrow tube, short in the anamniotes, elon- 
gate in the amniotes. No digestive glands are found in this region. 

The stomach. This portion of the digestive tract is dis- 
tinguished by its dilation (Fig. 120) into a large sac or series of 


sacs, and by the development of a thick wall of muscle from the 
splanchnic mesoderm in which it is enveloped. The stomach is 
rich in glands which aid in digesting the passing food. 

The intestine. All the regions of the digestive tract mentioned 
so far are derived from the fore-gut. The intestine is derived in 
part from the fore-gut, in part from the mid-gut, and in part from 
the hind-gut. It is impossible to indicate exactly which regions 
arise from these divisions of the gut, as both the fore-gut and the 
hind-gut expand at the expense of the mid-gut during the con- 
sumption of the yolk. As was said in the discussion of the de- 
velopment of body form, the division of the alimentary canal 
into these regions is the result of the method by which the head 
and tail are formed. The intestine becomes subdivided in various 
ways in the different groups, but we need notice only the most 
anterior of these, the duodenum, which is that portion of the 
intestine immediately succeeding the stomach and generally held 
to be derived from the fore-gut. The intestine is richly glandular 
throughout its length, but from the duodenum, in particular, we 
find developed two most important glands, the liver and the 
pancreas (Fig. 120). 

The liver. This gland arises from the ventral side of the duo- 
denum as an evagination which grows forward, expanding into 
a vesicle at the distal end and retaining its connection with the 
(i^denum by a narrow hollow stalk, the common bile duct, 
(Fig. 120). The sac-like distal end becomes subdivided, by the 
ingrowth of mesenchyme, into many tubules which often anasto- 
mose. In this process of growth and subdivision the liver grows 
about the vitelline veins (Chapter IX) and breaks these up into 
a system of hepatic capillaries. The cavity of the sac becomes 
the gall-bladder, to which the bile, formed in the glandular 
portion of the liver, is carried by means of the hepatic ducts. It 
releases these secretions into the duodenum via the common 
bile duct (ductus choledochus). 

The pancreas. This gland arises usually from three diver- 
ticula of the duodenum (Fig. 120), but the number of primordia 
is variable. One appears on the dorsal side of the duodenum 
just posterior to the stomach; the others arise on the ventral 
side, usually in connection with the hepatic diverticulum. The 
primordia increase in size, and break up into masses of secretory 


tubules at the distal end of each. The primordia unite and their 
proximal ends become the pancreatic ducts, one or more of which 
may be suppressed in later organogeny. The pancreas, as well 
as elaborating a digestive pancreatic juice discharged through the 
pancreatic duct, forms a hormone (insulin), which is carried away 
by the blood stream. It functions therefore as an endocrine 
gland in addition to its digestive function. Insulin, as is well 
known, is important in the treatment of diabetes. 

The cloaca. The intestine behind the duodenum is variously 
subdivided in the different vertebrate classes, but all are alike in 
the possession of a terminal region which receives in addition the 
ends of the nephric ducts and of the genital ducts (see Chapter IX). 
From the cloaca also arises the urinary bladder and the allantois 
of the amniotes. 

The cloaca, like the pharynx, communicates with the exterior 
by means of an aperture lined with ectoderm, which arises as 
a median ventral pit, the proctodeum (Fig, 118), just in front 
of the tail region. The proctodeum is formed at the point where 
the blastopore was obliterated and is separated from the hind-gut 
temporarily by means of the cloacal plate, which is comparable 
with the oral plate. For a time there is a blind pocket of endo- 
derm posterior to the cloaca, which is known as the postcloacal 
gut. The region of the cloaca anterior to the entrance of the 
nephric ducts is known as the rectum; its aperture is called the 
vent. In mammals the rectum becomes separated from the re- 
mainder of the cloaca, which is then known as the urogenital 
sinus. Each of these cavities has a separate exit, the two openings 
being the anus and the urogenital aperture, respectively. 

THE FROG (SEE ALSO CHAPTER XI). The mouth of the tad- 
pole does not open until a few days after hatching. It remains 
round during larval life and is enclosed by the mandibular ridges. 
Outside these, folds of ectoderm project as the larval lips, on which 
horny larval teeth develop. These larval structures are lost at 
metamorphosis, when the definitive jaws and teeth are formed 
in the usual way. The tongue is compound, arising from a pri- 
mary tongue and a gland field, relatively late in larval life. The 
hypophysis is solid (Fig. 181). 

Six visceral pouches appear, of which the first never ^becomes 
perforated, its closing plate becoming the tympanum of the ear, 


and its cavity persisting as the tubo-tympanic cavity. Of the 
five remaining pouches, the second and third open to the ex- 
terior before the first and fourth, and the fifth remains vestigial. 
External gills appear on the third, fourth, and fifth arches (that 
on the fifth arch being rudimentary), but are resorbed later when 
covered by the operculum. This structure fuses with the body 
surface on the right side, but on the left it opens to the exterior 
by an opercular aperture. The internal gills appear as demi- 
branchs commencing on the anterior side of the third arch. The 
first three gills, therefore, have two demibranchs, while the fourth 
has but one, formed from the anterior side of the sixth arch. The 
visceral clefts, gills, and opercular cavity are lost as separate 
structures by cell proliferation and reorganization just before 
metamorphosis. The lungs appear early in larval life as solid 
primordia of the pharynx. These acquire cavities prior to the 
formation of the tracheal groove which is relatively late in forma- 
tion. The thyroid arises, just before hatching, as a solid diver- 
ticulum of the pharynx; it soon detaches itself and divides into 
two bodies which later become vesicular. The two thymus 
glands are formed from epithelial bodies on the dorsal side of the 
first and second visceral pouches. Epithelial bodies arise from 
the ventral sides of the second visceral pouches. It has been 
claimed that those of the third and fourth pouches become the 
carotid glands. The sixth pharyngeal pouches give rise to the 
ultimobranchial (suprapericardial) bodies. (Fig. 121A.) 

The esophagus is short, and the stomach a simple dilation. 
The liver arises as a backward ventral diverticulum of the duo- 
denum (Fig. 181). All three pancreatic primordia appear and 
fuse; the dorsal duct disappears, while the two ventral ducts 
fuse to become the adult pancreatic duct. The intestine of the 
tadpole,, which is long and coiled (about nine times the body 
length), becomes resorbed during metamorphosis until it is about 
one-third of its larval length (Fig. 122). 

The postcloacal gut loses its connection with the neural tube 
(neurenteric canal) during the backward growth of the tail. The 
urinary bladder does not appear until after metamorphosis. 

THE CHICK (SEE ALSO CHAPTER xii). The mouth opens on the 
third day of incubation. The teeth are represented only by the 
tooth ridges which are the first stage in the appearance of the 


enamel organs. These appear on the sixth day of incubation and 
disappear shortly after the cornification of the jaws. This re- 
sults in the formation of the beak and the egg tooth, the latter 
a horny projection on the upper jaw which is used in breaking 
through the shell at the time of hatching, and soon after dis- 
appears. The primordia of the tongue appear on the fourth day. 
Five visceral pouches appear, of which the first three open to 
the exterior during the third day of incubation (Fig. 218). The 



FIG. 122. Digestive tube in A, tadpole, and B, frog, to show actual shortening of 
intestine. (After Leuckart wall-charts.) 

first cleft closes during the fourth day, and the dorsal part of the 
pouch becomes the tubo-tympanic cavity. With the extension 
of the cervical flexure, the remaining pouches are crowded to- 
gether and disappear. The thyroid appears on the second day, 
separates from the pharynx on the fourth, and on the seventh 
divides into two bodies which migrate backward to the junction 
of the common carotid and subclavian arteries. The thymus 
arises from the dorsal epithelial bodies of the third and fourth 
visceral pouches, while the parathyroid rudiments arise from 



the ventral epithelial bodies. The fifth pouch gives rise to the 
ultimobranchial bodies. The lung primordia (Fig. 123) appear on 
the third day and grow back, becoming surrounded by mesen- 
chyme. The primary bronchi subdivide to form a respiratory 
tree, some branches of which extend among the viscera and even 
into the hollow bones, as the accessory air sacs. 

The esophagus is relatively long; and a dilation, the crop, 
forms at its posterior end. The stomach is divided into an ante- 
rior proventriculus, which contains the gastric glands, and a 

Visceral arches 

Pharynx>. HI n 



Yolk stalk 


FIG. 123. Endodermal derivatives in a 72-hour chick. 

muscular gizzard at the posterior end. The liver primordium 
arises at the edge of the anterior intestinal portal on the second 
day and, therefore, presents the aspect of an anterior ventral 
and two posterior lateral diverticula for a short time. These 
fuse, however, by the end of that day, as the backward extension 
of the fore-gut continues. Three pancreatic diverticula are 
formed, the dorsal one on the third day, the ventral ones on the 
fourth. They fuse in later development, and either two or three 
of the ducts persist. The anterior portion of the mid-gut becomes 
the small intestine, the large intestine arising from the posterior 

MAN 191 

The cloaca is first distinguishable on the fourth day, when the 
proctodeum also is first apparent. The cloaca is ultimately 
divided into three regions: an anterior portion, the coprodeum, 
into which the rectum enters; an intermediate part, the urodeum, 
into which the nephric ducts and gonoducts enter; and the ter- 
minal proctodeum. 

MAN (SEE ALSO CHAPTER XIII). The mouth opens in the sec- 
ond or third week, and, like that of all vertebrates, develops lips 
(fifth week). Ten teeth papillae and enamel caps, the primordia 
of the milk teeth, appear in each jaw. This is a long-drawn-out 
process, the germs of the third molar not appearing until the 
fifth year of infancy. The tongue arises from swellings on the 
first three arches, the secondary tongue, or gland field, appearing 
as the tuberculum impar, which does not, however, appear to 
contribute to the ultimate structure of the tongue. 

Five pairs of visceral pouches appear, none of which becomes 
perforated. The first gives rise to the tubo-tympanic cavity. 
The ventral portion of the second persists as the fossa in which 
the tonsil develops. The dorsal epithelial bodies from the third 
and fourth pair of pouches become the parathyroids. The 
ventral epithelial bodies of the third pair of pouches unite to form 
the thymus gland. Similar bodies from the fourth pair may give 
rise to vestigial thymus-like bodies which remain attached to 
the parathyroids from the same pouch. The fifth pair become 
the ultimobranchial bodies. The thyroid gland undergoes an 
incomplete division into two lobes which remain connected by a 
narrow isthmus. The lungs (Fig. 124) arise toward the end of the 
fourth week, from a laryngo-tracheal groove. The cartilages and 
musculature of the larynx arise from the branchial arches. 

The esophagus, at first relatively short, lengthens as the back- 
ward movement of the heart and lungs displaces the stomach. 
The latter organ arises as a dilation of the fore-gut posterior 
to the esophagus. Continued growth, mainly on the dorsal 
surface, produces the greater curvature, and a displacement of 
the whole organ so that the cephalic end is moved to the left 
and the caudal end to the right. This is followed by a rotation 
of theyiomach on its long axis through 90 to the left. The liver 
ariipuring the third week as a ventral groove in the duodenum. 
The^ancreas appears slightly later, with either two or three 



primordia according to whether or not one of the ventral primordia 
is suppressed. The ventral pancreatic duct persists and opens 
into the common bile duct. The point of division between small 
and large intestines is marked by the formation of a blind pouch, 

Visceral arches 




( Mesonephric duct ) 

( Metanephric 
diverticulum > 

FIG. 124. Endodermal derivatives in 10-mm. pig. (From a wax reconstruction 
by G. W. Hunter and L. T. Brown.) 

the cecum. The distal end of the cecum does not grow as rapidly 
as the proximal region and so remains a finger-like projection 
known as the vermiform appendix. The small intestine, growing 
more rapidly than the large, is thrown into a set of six primary 
coils, each of which develops secondary coils. 
The cloaca becomes divided, by a frontal partition, into a 



dorsal rectum and a ventral urogenital sinus. The cloacal 
membrane is correspondingly divided into a rectal and a uro- 
genital plate, and the final openings are the anus and the uro- 
genital aperture. The urogenital sinus later is divided into a 
phallic portion (see page 211) and a vesico-urethral portion. The 
latter gives rise to the urinary bladder at its distal end, and to 
the urethra at its proximal end. 


The endoderm gives rise to the epithelial lining of the following 
structures : 

A. Fore-gut 

I. Oral cavity (also partly from ectoderm of stomodeum) 

Teeth (also partly from ectoderm) 

IL Pharynx 

Trachea and lungs 


Visceral pouches 

Auditory tube and chamber 
Fossa of palatine tonsil 
Ultimobranchial bodies 

III. Esophagus 

IV. Stomach 
V. Duodenum 


B. Mid-gut 

I. Intestine 

C. Hind-gut 

I. Cloaca (also partly from ectoderm of proctodeum) 

Urogenital sinus 
Urinary bladder 

Urethra (also partly from mesoderm, page 




Developmental Anatomy, 3rd Ed., Chaps. 7 and 8. 

Traite* d'embryologie des verte*bre*s, Part 2, Bk. 1, Chap. 5; 

L. B. 1934. 
Brachet, A. 1921. 
Bk. 2, Chap. 4. 
Hertwig, 0. 1906. Handbuch, Vol. 2, Chaps. 1, 2, and 4. 
Keibel and Mall. 1910-1912. Human Embryology, Chap. 17. 
Kellicott, W. E. 1913. Chordate Development. 
Kerr, J. G. 1919. Textbook of Embryology, Vol. II, Chap. 3. 
Kingsley, J. S. 1926. Comparative Anatomy of Vertebrates, 3rd Ed. 
Lillie, F. R. 1919. The Development of the Chick, 2nd Ed. 
McMurrich, J. P. 1923. The Development of the Human Body, 7th Ed. 


The middle germ layer arises as three different aggregates of 
cells between the ectoderm and endoderm: the notochord; the 
mesoderm; and the mesenchyrne. The origin of the notochord 
has already been described, and its later history will be discussed 
in connection with the skeleton. Organs of mesenchymatous 
origin will be taken up in connectiqn with the history of the 
region from which their mesenchyme originates. Of the struc- 
tures derived from the mesoderm, we shall consider first those 
arising from the lateral mesoderm, then those whose origin is from 
the intermediate mesoderm, and finally those derived from the 
axial mesoderm. 


Cavities may appear in all three divisions of the mesoderm; 
if in the myotomes, they are known as myocoels; if in the 
nephro tomes, they are called nephrocoels; the cavity of the 
lateral mesoderm is the coelom (Fig. 76). In some forms the 
three cavities are confluent. The connection, however, is a tem- 
porary one, and the myocoels soon disappear. In other forms 
they make a transitory appearance and are entirely disconnected 
with the other cavities, and in many vertebrates myocoels are 
never formed. The nephrocoels will be considered with the 
nephric organs. The coelom in amphioxus has a metameric 
origin from the ventral portions of the enterocoels, which become 
coiifluent'lil'TIiis point by the disappearance of the intervening 
anterior and posterior partitions. In vertebrates the coelomic 
cavity arises from the splitting of the lateral mesoderm into a 
dorsal somatic and a ventral splanchnic layer. In the amniotes 
this split continues out into the extra-embryonic mesoderm, thus 
giving rise to the exocoel, or cavity of the chorion. The coelom 
does not extend anterior to the visceral arches. Transitory 
cavities have been found in the arches and, indeed, in the head 



itself, and these have been interpreted as the remains of a cephalic 
coelom. It will appear later that these are more probably the 
rudiments of cephalic myotomes. The coelom does not extend 
into the tail. 

Somatopleure and splanchnopleure. The somatopleure has 
already been defined as the outer layer of the lateral mesoderm 
together with the ectoderm with which it becomes associated. 
Between these two there is an invasion of mesenchymatous cells 
from the dermatomes and myotomes which give rise to the 
corium of the skin (see Chapter X) and to its dermal muscu- 
lature (see page 239). The somatic mesoderm lining the outer 
wall of the coelom becomes the outer peritoneal lining. The 
splanchnopleure is the innr layer of the lateral mesoderm plus 
the endoderm with which it is associated. Between these two 
occurs a migration of mesenchyme cells which give rise to the 
splanchnic musculature and blood vessels, while the splanchnic 
mesoderm itself forms the inner peritoneal lining of the coelom. 

The mesenteries (Fig. 125). In all the vertebrates, the 
coelom is divided for a time into right and left halves by sagittal 
partitions above and below the alimentary canal, known as the 
dorsal mesentery and the ventral mesentery, respectively. These 
are formed by the inward growth of the splanchnic mesoderm 
above and below the digestive tube and the subsequent fusion 
of these sheets in the median line. The ventral mesentery dis- 
appears posterior to the liver, probably in connection with the 
coiling of the intestine. The dorsal mesentery (Fig. 125) persists 
as the support of the alimentary canal, and frequently becomes 
subdivided into regions which are named from the supported 
organ, such as the mesogastrium which supports the stomach, the 
mesoduodenum, etc. In the formation of the ventral mesentery, 
two organs, the heart and the liver, owing to their ventral position, 
are caught in between the two advancing sheets of splanchnic 
mesoderm. In these regions, therefore, the ventral mesentery is 
divided into an upper and a lower half. The ventral mesentery 
dorsal to the heart becomes the dorsal mesocardium; that part 
which is ventral to the heart is the ventral mesocardium (Fig. 
126A). Both eventually disappear as the heart increases in size 
and complexity. In the region of the liver, the dorsal half of the 
mesentery becomes the dorsal mesohepar, while the ventral por- 




Pericardial cavity, 
Ventricle of heart 

Ventral mesocardium 


Ventral mesentery 
(falciform ligament 

Dorsal mesocar- 

-Septum transversum 



Ventral mesentery 

(lesser omentum) 

Dorsal mesogastrium 

Dorsal pancreas 



FIG. 125. Diagram of mesenteries in early human embryo from left side. A, B, 
and C indicate planes of sections shown in Fig. 126. (From Arey after Prentiss.) 

Neural tube 





Neural tube 



Aorta Notochord-^^ 

Postcar- ,,,_, ZZZZZZ 
dinalvein Aom 
Dorsal mesentery,. 






f alciform 

FIG. 126. Diagrams of mesenteries in early human embryo as seen in transverse 
sections. Compare Fig. 125. (From Arey after Prentiss.) 



tion is the ventral mesohepar (Fig. 126B). The primordia of the 
pancreas lie originally in the dorsal and ventral mesenteries, 
respectively, but with the rotation of the stomach all are included 
in the dorsal mesentery. The peritoneal supports of the nephric 
and genital organs will be considered in the following section. 
The spleen (see page 224) arises in the mesogastrium, close to the 
wall of the alimentary canal, and is probably mesodermal in 

Later divisions of the coelom. The coelom becomes divided 
into an anterior pericardial cavity surrounding the heart, and 
a posterior abdominal cavity surrounding the viscera, by the 
septum transversum, a transverse partition which grows out 
from the bridge of mesoderm surrounding the vitelline veins 






Abdomina 1 

FIG. 127. Diagrams of coelom and its divisions in A, fish, B, amphibia, reptiles 
and birds, and C, mammals. (After Kingsley.) 

where they cross the coelom en route from the body wall to the 
heart (Fig. 127A). These cavities are connected during a large 
part of the embryonic period by pericardio-peritoneal canals 
where the septum has failed to unite with the ventral body wall. 
In the amniotes, additional septa develop behind the lungs and 
separate the pleural cavities, which contain the lungs, from the 
remainder of the abdominal cavity, which is now known as the 
peritoneal cavity (Fig. 127B). The pleural cavities are separated 
from each other in the median line by the mediastinum. In the 
mammals (Fig. 127C) the partition separating the lungs from the 
viscera receives musculature from the myotomes and becomes the 

THE FROG (SEE ALSO CHAPTER XI.) In the frog, the ventral 
mesentery disappears as soon as it has been formed, except in the 
region of the heart and liver. The ventral mesocardium appears 


before the dorsal mesocardium is formed, and disappears soon 
after, to be followed by the disappearance of the dorsal meso- 
cardium. The ventral mesohepar also has but a short period of 
existence. The septum transversum receives much of its sub- 
stance from the mesoderrnal sheath of the liver. No pleural 
cavities are formed. 

THE CHICK (SEE ALSO CHAPTER XII.) In the chick, both dor- 
sal and ventral mesenteries are formed. The latter, however, per- 
sists only in the region of the fore-gut, and gives rise to the 
mesocardia, which soon disappear; the dorsal mesohepar, which 
becomes the gastro-hepatic omentum, and the ventral mesohe- 
par, which becomes the falciform ligament. The septum trans- 
versum is not completed until the eighth day of incubation. 
The pleural cavities are cut off from the pericardial cavities by 
a pleuro-pericardial septum, and from the peritoneal cavity by 
the pleuro-peritoneal septum. 

MAN (SEE ALSO CHAPTER xiil). From the first, the peri- 
cardial cavity is distinguishable from the abdominal cavity, 
inasmuch as it never communicates directly with the extra- 
embryonic coelom as does the abdominal cavity. As in the chick, 
its posterior boundary is coterminous with that of the fore-gut, 
but it is in communication with the abdominal cavity by means 
of the parietal recesses, passages which correspond to the peri- 
toneo-pericardial canals of the anamniotes. The recesses are 
divided frontally by the vitelline veins into dorsal and ventral 
parietal recesses. With the formation of the septum transversum, 
the ventral recesses are incorporated into the pericardial cavity. 
The dorsal recesses become the pleural cavities; and the pleuro- 
peritoneal septum, which divides them from the peritoneal 
cavity, is formed by the upward growth of the diaphragm. The 
musculature of this organ arises from the fourth cervical myotome 
during the backward growth of the diaphragm. The rotation of 
the stomach results in a rearrangement of the mesenteries, for 
an account of which the reader is referred to Hertwig or Keibel 
and Mall. 


The nephric or excretory system of vertebrates is essentially 
a paired series of tubes (nephridia), developed in the intermediate 
mesoderm, which collect nitrogenous wastes from the blood and 



..-;." Mesonephroe 


\Cr.o-;^ : 



Mesonephric duct 

Metanephric duct 

discharge them to the exterior by two longitudinal ducts emptying 
into the cloaca. The intermediate mesoderm in the anterior 

part of the body is divided into 
nephrotomes corresponding to the 
somites. There are three different 
types of kidneys among the verte- 
brates (Fig. 128). The first is the 
pronephros, which arises from the 
anterior nephrotomes and is the 
functional kidney in the larval 
stages of the fish and the amphib- 
ians. The second is the meso- 
nephros, which arises from nephro- 
tomes posterior to the pronephros 
and is the functional kidney of 
adult anamniotes and embryonic or 
fetal amniotes. The third is the 
metanephros which is the functional 
kidney of adult amniotes. 
FIG. 128. Diagram to show rela- The pronephros. This organ is 

tionships of vertebrate excretory formed during development by all 
systems. . l J 

vertebrates, but is best developed 

in larval types like the frog, where it arises from nephrocoels 
(Fig. 129) in the anterior nephrotomes (III, IV, V), the dorsal 
ends of which grow caudally and unite with each other to form 
the pronephric duct which grows backward toward the cloaca. 
The ventral ends of the nephrocoels open into the coelom, and 
these openings, the nephrostomes, become lined with long cilia. 
The tubules meantime elongate and become contorted as they pro- 
ject into the surrounding posterior cardinal vein. Median to each 
nephrostome, the splanchnic mesoderm bulges out and in this 
projection develops a net of capillaries, or glomerulus, which be- 
comes connected with the dorsal aorta. The pronephros is func- 
tional, at most, for a short time ; and it disappears as the meso- 
nephros develops to replace it. 

The mesonephros. The mesonephros, like the pronephros, 
is developed by all vertebrates. It becomes the adult kidney 
of the anamniotes, but is functional during the embryonic (and 
fetal) period only of the amniotes. Portions of the mesonephros 



become associated with the genital organs of the adult (see next 

The mesonephros also develops as a series of segmental nephro- 
coels, but in the nephrotomes posterior to those containing the 



Fia. 129. Diagrams showing three stages in the development of the pronephric 

tubule. (After Felix.) 

pronephric ducts with which they unite (Fig. 130). After the 
degeneration of the pronephros, the tube is known as the meso- 
nephric or Wolffian duct. The ventral ends of the nephrocoels 
acquire coelomic nephrostomes in the anamniotes. In amniote 
development, nephrostomes are seldom formed. A glomerulus 
connected with the dorsal aorta and the cardinal veins arises in 
connection with each tubule, as in the pronephros. An important 
difference between the pronephros and the mesonephros lies in 
the fact that the number of nephric tubules in each nephrotome 
is greater in the mesonephros (Fig. 131). These arise by the 
constriction of the posterior median part of each nephrocoel into 
a small vesicle which gives rise to a secondary tubule; each of 
these tubules acquires a glomerulus and nephrostome at the 



Anlage of 
mesonephric tubule* 

proximal end. The con- 
nection of these secondary 
tubules with the Wolffian 
duct, however, is attained 
by an evagination from the 
duct itself which grows out 
as the collecting duct to 
meet the developing 
secondary tubule. From 
these secondary tubules, 
tertiary ones bud off and 
develop in like manner, 
acquiring connections with 
the collecting duct through 
an evagination of this 
canal. As many as eight 
tubules may be formed in 
a single segment by this 
process of budding. This 
complexity is greatest at 
the posterior end of the 
mesonephros. In the am- 
Bowman's capsule niotes, the mesonephros 

FIG. 130. Diagrams showing four stages in (except for that portion 
development of mesonephric tubule. (From associated with the genital 
Arey after Felix.) organg) ^p^ aftef 

the metanephros has 

been formed. ^ 

The metanephros. 
The metanephros, 
which is found as a 
separate kidney only 
in adult amniotes, 
probably is equiva- 
lent to the posterior 
portion of the meso- Fia 131 - ~~ Digram to show origin of secondary and 
ko ^f V,^ o^ tertiary mesonephric tubules from primary tubules. 

nepnros 01 tne anam- (After Brauer ) 
niotes, which it re- 
sembles greatly in its organogeny. 


Iry 3ry 2ry 


Iry 2ry duct 






The region in which the metanephros arises is, like that in 
which the earlier kidneys are found, the intermediate mesoderm. 
But in the posterior region of the body this mass is never seg- 
mented into separate nephrotomes. The first indication of meta- 
i?ephros formation is the appearance of an evagination from the 
dorsal surface of the mesonephric duct near the point at which 
the latter enters the cloaca. This evagination grows dorsally and 

FIG. 132. Diagrams to show origin and development of metanephric tubules. 
Collecting tubule in center, secretory tubules to right and left, the one on the right 
relatively more advanced. (From Arey after Huber.) 

then turns forward to become the metanephric duct, or ureter, in 
much the same manner as the collecting ducts of the mesonephros 
arose. The metanephric duct then sends out into the nephrog- 
enous tissue evaginations which increase in length and branch 
repeatedly to form the collecting tubules of the metanephros. 
Around the distal end of each tubule, a small mass of the nephrog- 
enous tissue condenses and acquires a lumen like the nephrocoels 
of the pronephros and mesonephros (Fig. 132; 1,8). From these 
vesicles the secretory nephric tubules arise by a process of elonga- 


tion and later fuse with the collecting tubules just described 
(Fig. 132; 3, 4)* I* 1 each of the tubules a capsule develops for the 
reception of a glomerulus which later acquires a connection with a 
branch of the renal artery (Fig. 132; 4) 5}. Development pro- 
ceeds from the posterior end toward the anterior, instead of in the 
opposite direction as in the earlier types of kidneys. The portion 
of the Wolffian duct nearest to the cloaca is absorbed by it so that 
the ureter has an opening into the cloaca separate from that of the 
mesonephric duct. From the region of the cloaca into which the 
ureters open is formed the urinary bladder and urethra (page 
193). In mammals, at least, the enlarging bladder includes part 
of the ureter. 

The later history of the kidneys and their ducts is considered 
in the next section. 

THE FROG (SEE ALSO CHAPTER XI). Three pronephric tubules 
are formed (somites II, III, IV), each with a nephrostome. The 
region of the coelom into which these open is cut off ventrally 
by the development of the lungs and becomes the pronephric 
chamber. The glomeruli soon unite to form a glomus. Before 
metamorphosis the pronephric tubes, and that portion of the 
duct into which they open, degenerate. 

Mesonephric tubules appear in the nephrotomes (somites VII- 
XII). These have nephrostomes in early larval life; but at the 
time the pronephroi degenerate the portion of each mesonephric 
tubule next to the nephrostome (peritoneal canal) breaks away 
from the remainder of the tubule and fuses with the posterior 
cardinal vein. The mesonephros is the functional kidney of the 
adult, and the Wolffian duct, therefore, functions as the ureter. 

THE CHICK (SEE ALSO CHAPTER xii). About twelve proneph- 
ric tubules arise (somites V-XVI) beginning on the second day of 
incubation. Nephrostomes are formed, but glomeruli do not 
appear until the third and fourth days of incubation, at which 
time the pronephros is degenerating. The pronephric duct arises 
at the ninth somite. 

Mesonephric tubules arise from the intermediate mesoderm 
between somites XII arid XXX, the more anterior of which 
develop nephrostomes. The main part of the mesonephros, how- 
ever, arises between somites XX and XXX, where the con- 
tinued growth of the tubules causes the projection of this region 


into the coelom as the Wolffian body. It is extremely doubtful 
whether the mesonephros ever functions as a kidney, as it begins 
to degenerate on the eleventh day. 

The metanephros arises on the fourth day of incubation, from 
two primordia as usual, the intermediate mesoderm in somites 
XXXI-XXXIII, and an evagination of the mesonephric duct, 
comparable to the collecting ducts of the mesonephric tubules, 
which becomes the ureter, pelvis, and collecting ducts. 

MAN (SEE ALSO CHAPTER xiii). Pronephric tubules arise in 
somites VII-XIII, develop nephrostornes and glomeruli, but 
degenerate rapidly. 

Mesonephric tubules appear in the intermediate mesoderm 
between the sixth cervical and fourth lumbar segments, but 
those of the cervical and thoracic segments soon degenerate. 
Nephrostomes are formed by the more anterior tubules but have 
only a transitory existence. The mesonephros does not function 
as a kidney. 

The metanephros has a double origin as in the chick. 


The genital organs may be grouped into two classes: (1) the 
primary genital organs, or gonads, in which the germ cells de- 
velop ; and (2) the accessory genital organs, whose original func- 
tion is the discharge of the germ cells from the body. 

The gonads consist of the germ cells and the subordinate tissues, 
blood vessels, nerves, connective tissue, etc., which make up a 
large part of these glands. In an earlier chapter it has been 
shown that the primordial germ cells may first appear in the 
endoderm of the gut wall and thence migrate by way of the 
splanchnic mesoderm, dorsal mesentery, and peritoneum to their 
definitive position in a thickening of the peritoneum on the 
mesial side of the nephrotomes. This thickening is called the 
genital ridge (Fig. 133B). A considerable body of evidence is 
accumulating to indicate that germ cells may also arise from the 
cells of the genital ridge itself. 

The genital ridge is now invaded by mesenchymal cells, and 
projects into the coelomic cavity. In some amphibians, a meta- 
meric arrangement corresponding to the myotomes has been 
recorded, but following this the segments unite. The anterior 



and posterior ends of the ridge degenerate, and the middle por- 
tion enlarges and is separated by a longitudinal groove from the 
mesonephros so that it hangs in the coelom suspended by a fold 
of the peritoneum, known as the mesorchium in the male or the 
mesovarium in the female. The germ cells have by this time 
become transformed into gonia (Chapter III) and the germ glands 
are known as gonads. 

Within the gonads, the gonia come to lie in nests, close to 
the germinal epithelium. Tubular outgrowths from the nephric 







FIG. 133. Diagrams to show early development of the gonads in transverse sec- 
tions. A, testis. B, genital ridge. C, ovary. (After Corning.) 

tubules of the mesonephros approach these nests. The later 
history of the gonads differs in the two sexes. 

Testis. In the male, the nests of spermatogonia become 
tubules which connect with the tubules growing in from the 
mesonephros (Fig. 133A). The testicular parts of these canals 
are known as the seminiferous tubules, the nephric portions as 
the efferent ductules. The walls of the seminiferous tubules are 
composed of spermatogonia and sustentacular cells which act 
as nurse cells to the developing sperm. Between the tubules lie 
partitions of mesenchyme which make up the stroma of the testis 
and contain the interstitial cells, which are supposed to be con- 
cerned in the formation of the male hormone. It is because of the 
presence of these cells that the testis is sometimes spoken of as 
the " interstitial gland." It is now well established that the 
testis secretes a " male " hormone whose presence in the blood 
has much to do with the male secondary characters. Eventually, 
the tubules become separated from the surrounding germinal 



epithelium by the development of a mesenchymatous layer called 
the tunica albuginea. 

Ovary. In the female, the nests of oogonia become separate 
follicles (Fig. 133C) which never attain connection with the meso- 
nephric tubules. These tubules consequently degenerate. A 
follicle consists of a single oogonium surrounded by follicle cells 
which may be compared to the sustentacular cells of the male. 
In the mammalian ovary the primary follicle is greatly enlarged 
to form a vesicular (Graafian) follicle (Fig. 134), which protrudes 

Stratum grannlowm 

FIG. 134. Section of human vesicular (Graafian) follicle. 


(From Arey after 

from the surface of the ovary. The follicle cells multiply and 
secrete a follicular fluid which presses the outer wall (stratum 
granulosum) away from the egg and a layer of follicle cells 
immediately surrounding it. These form a projection (cumulus 
oophorus) into the cavity of the follicle. When ovulation takes 
place the wall of the follicle is ruptured, and the egg, still sur- 
rounded by its investment of follicle cells, now known as the 
coroSjraffia|a (page 41 ) , isjwashed put with the^fo^ 
After ovulation the follicle cells enlarge, multiply, and secrete a 
yellow^substance, the whole forming a corpus luteum. Hisaw 
has identified hormones from corpus luteum which produce 


definite effects on the uterus and other parts of the female body 
associated with pregnancy and parturition. The existence of 
female hormones formed in the ovary is now definitely proved. 
These hormones appear to be formed in the follicles and to be 
quite distinct from the hormones derived from the corpus luteum 
(Hisaw). The tunica albuginea of the ovary develops much later 
than that of the testis but is also of mesenchymal origin. 

The genital ducts. The sperms formed in the seminiferous 
tubules of the testis are discharged into the mesonephric tubules 
and thence make their way into the mesonephric duct, which 
accordingly becomes the male genital duct. The ova, on the 
other hand, are discharged directly into the cavity of the coelom 
whence they are received into a new tube, the oviduct, by means 
of an opening, the ostium tubae (abdominale). The mesonephric 
duct is often called the Wolffian duct; the oviduct is frequently 
called the Mullerian duct. Both ducts appear in every embryo 
(Fig. 135 A), but the later histories of the two differ according to 
the sex. 

The Wolffian duct. In the male (Fig. 135B), the efferent 
ductules toward the posterior end of the series become occluded, 
leaving only a few at the anterior end functional. These lose 
their renal corpuscles and shorten greatly. In the amniotes, 
where the metanephros acts as the functional kidney, this anterior 
group becomes the epididymis, while the more posterior, non- 
functional vestige becomes the paradidymis. The mesonephric 
duct persists as the deferent duct. At the point where the defer- 
ent duct enters the cloaca, there develops a dilation, the seminal 
vesicle. In the female (Fig. 135C), the anterior portion of the 
mesonephros persists as the vestigial epoophoron, and the pos- 
terior portion becomes the paroophoron. Traces of the Wolffian 
duct sometimes persist, as in mammals, where this structure is 
known as Gartner's canal. 

The Mullerian duct. This canal arises in the elasmobranchs 
by the constriction of the pronephric duct into two tubes, of 
which the ventral becomes the Mullerian duct, while the dorsal 
tube becomes the Wolffian duct. The opening of the Mullerian 
duct into the coelom, the ostium tubae abdominale, is a persistent 
nephrostome. In all other vertebrates, this duct arises inde- 
pendently of and after the formation of the WolfSan duct, a 



fact possibly correlated with the delayed functioning of the ovi- 
duct as compared with the primary renal function of the Wolffian 
duct. In these vertebrates the duct arises in the mesoderm 
lateral to the Wolffian duct and grows both forward and back- 
ward until the abdominal and cloacal openings are formed. It 
is not formed until late in embryogenesis. In the female (Fig. 
135C), the posterior ends of the ducts are usually dilated as 



FIG. 135. Diagrams showing origin and early development of genital ducts. A, 
early stage showing mesonephros, gonads, (male on left, female on right) and 
duets. B, later stage in male, showing in broken lines the structures which de- 
generate. C, later stage in female. (After Felix.) 

storage chambers, and not infrequently fuse to form a uterus. 
In the male (Fig. 135B), the Miillerian duct degenerates, but 
vestiges are to be found even in the adult, such as the appendix 
testis and prostatic utricle of man, which represent the anterior 
and posterior ends of the female duct, respectively. 

Estrous cycle. Most vertebrates have an annual breeding 
season. Among the mammals, however, the fact that the young 
develop for a longer or shorter period (of gestation) in the uterus 
of the mother is associated with a periodical set of changes in the 


activity of the uterus which are known as the estrous cycle. 
There are three main stages : proestrum, estrus, and anestrum. 

During the proestrum the blood vessels of the uterine wall are 
congested, and in some animals (dog) there is destruction of the 
uterine wall accompanied by the discharge of blood into the 
cavity of the uterus. 

In estrus the destructive changes of the proestrum are repaired 
while the cavity itself often contains the secretions of the uterine 
glands and the materials discharged in the preceding period 
(" uterine milk "). It is in this period that ovulation usually 
takes place and the wall of the uterus is in the condition most 
favorable for the implantation of the blastocyst. The estrus re- 
ceives its name from the fact that this is the time in which the 
sexual drive is strongest. If implantation (page 140) and preg- 
nancy do not take place, a condition known as pseudopregnancy 
occurs in some animals (rat, rabbit, etc.). In the closing stages 
of the estrus, the wall of the uterus returns to its normal con- 
dition, accompanied in some animals (dog) by slight hemor- 
rhages. This period of repair is distinguished (Marshall) as the 

The estrus is succeeded by the anestrum, a name given to the 
interval lasting until the next proestrum commences. In many 
mammals estrus occurs but once during the breeding season, but 
in others it may take place more frequently. The period between 
each estrus and the next proestrum is sometimes known as a 
diestrum in these polyestrous species. 

There is a considerable difference of opinion among the authori- 
ties as to the exact relation between ovulation and menstruation, 
a term applied to the periodic hemorrhages characteristic of the 
female primate. It is assumed that the period of ovulation cor- 
responds to the estrus, but the clinical evidence is not clear as to 
whether the menstrual discharge is comparable to that of the 
proestrum or that of the closing stages of the estrus itself. 

The external genitalia. The genital organs so far considered 
are common to all vertebrates and are sometimes spoken of as the 
internal genitalia. External genitals are found only in those 
animals in which fertilization is internal. These organs serve the 
function of transmitting or receiving the sperm at the time of 
copulation. Internal fertilization is a phenomenon which has 



.Genital tubercle 

been observed in all classes of the vertebrates, but it is character- 
istic of all amniotes. 

Although the external genitalia differ in the sexes, they are 
embryologically homologous. Two types are recognized, duplex 
and simplex. In the duplex type, characteristic of the saurop- 
sids, sac-like extensions arise on each side of the cloaca, which 
in the male become the hemipenes or intromittent organ, while 
in the female they remain vestigial. 
In the simplex type, characteristic 
of mammals, a single median ecto- 
dermal prominence arises anterior 
to the cloacal aperture, to become 
the phallus (Fig. 136). In the 
male, the phallus enlarges and en- 
closes the greater part of the uro- 
genital sinus. In this way it be- 
comes the penis, while the enclosed FI G- 136. 

Sinus becomes the penile Urethra. r f 
i i ^ u Ti genitalia. 

In the female mammal, the phallus 
becomes the vestigial clitoris, while the sides of the urogenital 
sinus remain open as the labia minora which guard the opening 
of the urogenital vestibule. At the base of the phallus is a swel- 
ling, the genital tubercle, from which labio-scrotal folds arise on 
either side of the urogenital opening. In the male they fuse to 
form the scrotum, an external sac into which the testes descend; 
in the female they remain separate as the labia majora. 




Diagram to show the 
,he mammalian external 
(After Felix.) 










Ductus deferens 

(Wolffian) duct 

Gartner's canal 

Appendix testis 
Prostatic utricle 

Mlillerian duct 




Labia minora 


Labio-scrotal swellings 

Labia majora 


THE FROG (SEE ALSO CHAPTER xi). The genital ridges arise 
soon after hatching. Sex can be distinguished at the time when 
the embryo is about 30 mm. in body length. The anterior por- 
tion of each genital ridge degenerates and becomes a fat body. 

The Wolffian duct in the male acquires connection with the 
testis by means of some of the mesonephric tubules (vasa effer- 
entia), and serves as the deferent duct as well as the ureter. A 
seminal vesicle is formed. A rudimentary Mullerian duct ap- 
pears. In the female the Wolffian duct functions solely as a 
ureter while the Mullerian duct becomes the oviduct. 

No external genitalia are developed. 

THE CHICK (SEE ALSO CHAPTER xii). The genital ridge arises 
with the mesonephros as the urogenital ridge. Of this the 
anterior region gives rise to the gonad on the mesial side. Sex 
is not distinguishable until the seventh day of incubation. In 
the female, the right ovary develops only partially and finally 

The Wolffian duct becomes the deferent duct, connected with 
the testis by vasa efferentia forming the epididymis. The per- 
sisting mesonephric tubules of the posterior region of the meso- 
nephros form a paradidymis. In the female a vestigial epooph- 
oron and paroophoron represent these bodies respectively. The 
Mullerian ducts degenerate in the male without ever acquiring 
a cloacal exit. In the female the right Mullerian duct dis- 
appears while the left becomes the oviduct. The shell gland 
appears on the twelfth day of incubation, but the cloacal opening 
is not formed until the hen is six months old. 

No external genitalia are formed, although hemipenes are 
formed in some other birds. 

MAN (SEE ALSO CHAPTER xiii). The genital ridge arises on 
the mesial side of the mesonephros. Sex is not distinguishable 
until after the fifth week. 

Each Wolffian duct functions as a deferent duct, and both epi- 
didymus and paradidymis are formed, as is a seminal vesicle 
at the distal end. In the female, epoophoron and paroophoron 
are formed, while some portion of the duct itself may persist 
as Gartner's canal. The Mullerian ducts become the uterine 
tubes, which unite at their posterior ends to form the uterus 
and vagina. The latter is partially closed by a semicircular 



fold, the hymen, where it enters the urogenital sinus. In the 
male, vestiges of the anterior end of each Miillerian duct persist 
as the appendix testis, while the posterior end is represented by 
the rudimentary prostatic utricle. The dilation of the bladder 
results in the inclusion of the ureters (metanephric ducts) in 
its walls. The genital ducts (Wolffian or Miillerian ducts) empty 
into the urogenital sinus posterior to the bladder, in a region 
which constricts to form the urethra. About this develop a 
number of outgrowths which acquire cavities and form the pros- 
tate gland in the male, and the para-urethral glands of the female. 
The external genitalia are of the mammalian type. 


Closely associated with the nephric organs are the mesodermal 
interrenal glands, which frequently become associated with the 
suprarenal glands, of ectodermal origin, to form the so-called 





FIG. 137. Diagrams to show the origin of the suprarenal and interrenal components 
of the adrenal gland. A, origin as shown in cross section (after Corning). B, 
condition in amphibia. C, in birds. D, iiTmammals. 

adrenal glands. All are endocrine (or ductless) glands. The 
suprarenal portion of the adrenal forms the powerful hormone 
epinephrin (adrenalin) ; the interrenal portion secretes a hormone 
known as cortin (Swingle), which is employed in the treatment 
of Addison's disease. 



The interrenals. These arise as paired thickenings of the 
splanchnic mesoderm mesial to the nephrocoels. In some of 
the amphibians there are traces of a segmentation which is soon 
lost by fusion. There is no direct connection between the inter- 
renal and the mesonephros. These glands may fuse to form an 
elongate median organ or become associated with the suprarenals. 

The suprarenals. Although these glands are found in the 
vicinity of the mesonephros, they originate from the sympathetic 
ganglia (ectodermal) as described in the following chapter. They 
are separate structures in the fish, but unite with the interrenals 
in the tetrapods. 

The adrenals (Fig. 137). These compound glands are not 
found in the fish. In the amphibians the suprarenal portion of 
the gland is external to the interrenal portion. In the chick they 
are intermingled. In the amniotes, however, the interrenal sub- 
stance (cortex) surrounds the suprarenal (medulla). 


The vascular system is mesenchymatous in origin. It consists 
of separate cells, the blood corpuscles, floating in a fluid matrix, 

Blood island Ectoderm Somatic mesoderm Splanchnic mesoderm Bloodvessel Blood cells 

: ' , 


JttG. 138. Diagrams showing three stages in the development of capillary from 
blood island based on transverse sections of the area vasculosa in a seven somite 
chick. (From Arey.) 

the blood plasma, in a closed system of interconnected tubes, the 
blood vessels. Some vessels become lined externally with muscle 
fibers, and in one locality this muscular development gives rise 


to a pulsating heart by means of which the blood is kept in 

Origin of the blood-vascular system. The first indications of 
the vascular system are found in the splanchnopleure as blood 
islands (Fig. 138). In the telolecithal vertebrates this is always 
in the extra-embryonic splanchnopleure. These blood islands 
originate as local aggregates of mesenchyme. Later, the inner 
cells separate as corpuscles, while the outer ones form the endo- 
thelial lining of a vesicle. These vesicles anastomose with each 
other to form the extra-embryonic vitelline circulation. 

The blood corpuscles. The first corpuscles formed are the 
inner cells of the blood islands. Later corpuscles are budded off 

* A r </ - -: - - '- = - 

FIG. 139. Stages in the development of human red blood corpuscles. A, hemo- 
blasts. B, megaloblasts (anamniote type). C, D, normoblasts (sauropsid type). 
E, normoblasts in process of becoming F, ery throcytes. (From Arey after Prentiss. ) 

from the walls of the capillaries into their cavities. Mesen- 
chymal cells in regions where the capillary network is forming 
may develop into blood corpuscles and enter the blood stream. 
These first corpuscles are the hemoblasts (Fig. 139). 

Hemoblasts become differentiated into the different types of 
blood corpuscles in the following blood-forming centers: (1) 
the yolk sac; (2) the embryonic capillaries; (3) the liver, the 
spleen, and the lymph glands; (4) the bone marrow. In the 
adult the lymph glands give rise to lymphocytes, and the bone 
marrow to all types of corpuscles. 

The erythrocytes, or red corpuscles, are distinguished by the 
presence of hemoglobin which gives them their color. In the 


anamniotes the erythrocytes have a large vesicular nucleus with 
granular chromatin and a distinct cell membrane. In the sau- 
ropsida, the erythrocytes have a small compact nucleus. The 
mammalian erythrocyte is distinguished by the absence of the 
nucleus in the adult. In the development of mammals there is a 
succession of erythrocytes: first the anamniote type; then the 
sauropsid type; and finally the mammalian erythrocyte, which 
is produced by the extrusion of the nuclei from the blood cells 
of the sauropsid type (Fig. 139). 

The leucocytes, or white corpuscles, are of many types, for 
a discussion of which the reader is referred to the textbooks on 
histology. The preponderance of evidence indicates that these, 
like the erythrocytes, are derived from the hemoblasts. 

Origin of the intra-embryonic vessels. The first embryonic 
blood vessels (Fig. 140) are the vitelline veins which appear at the 
ventro-lateral margins of the fore-gut. These vessels unite in 
the region of the anterior intestinal portal to form the heart, then 
separate as the ventral aortae, which bend up around the pharynx 
in the mandibular arch as the first aortic arches, and continue 
backward as the dorsal aortae. These fuse at a very early stage as 
the dorsal aorta, from which branches are sent to each myotome 
and to the vitelline circulation. The posterior ends of the vitel- 
line veins fuse in small-yolked forms, such as the frog, to form a 
subintestinal vein which continues back to the tail. In large- 
yolked forms like the chick, the vitelline veins are widely separated 
and brought into connection only by the sinus terminalis which 
makes a circuit of the area vasculosa. The vitelline veins are 
the ventral venous channels of the splanchnic circulation. A 
dorsal set of vessels soon originates independently to form the 
somatic venous circulation. The first of these to appear are the 
anterior cardinal (precardinal) veins of the head. A similar pair, 
the posterior cardinal (postcardinal) veins, arise in connection 
with the nephric region. These, however, do not discharge their 
contents directly into the heart but into the anterior cardinals. 
The portions of the original anterior cardinals proximal to this 
juncture with the posterior cardinals are now called the common 
cardinal veins. 

The heart. Although the heart is primitively a paired organ, 
we have seen that the two primordia are soon fused into a single 




area vasculosa 

Aortic arches 




osterior cardinal vefri 
Anterior cardinal vein 
Common cardinal vein 


Internal carotid -f-l 
External carotid 

Ventral aorta 

Dorsal aorta 

Vitelline artery 

Anterior cardinal 

Common cardinal 

Vitelline vein 
Posterior cardinal 

FIG. 140. Diagrams to show fundamental plan of embryonic circulation. A, 
early stage in side view. B, later stage in side view. C, same from above, aortic 
roots pulled apart. 



median tube connected with the ventral aortae in front, and the 
vitelline veins (and later the common cardinals) behind. Around 
the endocardial lining there develops a coat of muscle fiber which 
later becomes striated to form the myocardium. Outside this 
is a lining of splanchnic rnesoderm which forms the egicardium, 
continuous with the lining of a part of the coelom surrbun3ing 
the heart, which will later be cut off by the septum transversum 
to form the pericardium. In this the heart is suspended by a 
dorsal and a ventral mesentery known respectively as the dorsal 
and ventral mespcardia. 

The later history of the heart is one of growth and subdivision 
into special chambers. Because the local growth of the heart 
is limited by the anterior and posterior walls of the pericardium 


FIG. 141. Diagrams to show early stages in development of vertebrate heart. 
A, paired heart tubes. B, same fused. C, primary flexure. D, later "S" stage. 
E, after antero-dorsal displacement of atrium. 

and by the mesocardia in which it is suspended, any extension 
in length must be accompanied by coiling, The primary flexure 
of the heart is toward the right, thus changing the shape of the 
organ from a straight tube to a C-shaped one. Further growth 
results in the twisting of the heart into the shape of an S. Still 
later, the original posterior loop of the S is pushed forward and 
dorsad so that it comes to lie above the morphologically anterior 
end (Fig. 141). 

The original chambers of the heart are produced by local 
dilations, of which the most posterior is the sinus venosus; next to 
this is the atrium; in front of this, the ventricle; and finally, 
the bulbus arteriosus. The sinus is the chamber into which the 
primitive veins enter; the atrium is a thin- walled distensile 



chamber; the ventricle is a thick- walled, muscular, pulsating 
pump; and the bulbus is the chamber from which the blood 
eSD^fs the grimitive arteries. 

These chambers undergo different changes in the various types 
of vertebrates. Of these, the most important is a progressive 
differentiation, completed in the mammals and birds, of the 
atrium and ventricle into separate right and left halves, of which 
the right side receives venous blood from all parts of the body 
and transmits it to the lungs for respiratory exchange. From 
the lungs the blood is returned to the left side of the heart and 
thence conveyed to all parts of the body. 

The arteries (Fig. 142). The ventral aortae fuse into a single 
median tube sending branches into each of the visceral arches. 

Coehac Anterior 

mesenteric Posterior 


FIG. 142. Diagrams to show principal arteries; A, in side view, B, cross section 

through mesenteric. 

These branches, which unite with the dorsal aortae, are usually 
six in number and are known as the aortic arches. Anterior to 
these the ventral aortae continue forward as the external carotid 
arteries. Similar forward extensions of the dorsal aortae are 
known as the internal carotid arteries. In the region of the 
aortic arches the dorsal arteries remain separate as the aortic 
roots (radices aortae). Behind them, as has been mentioned, the 
paired vessels fuse as the median dorsal aorta. 



The aortic arches. In larvae breathing by means of external 
gills, a loop from each aortic arch grows out into the gill develop- 
ing on the visceral arch with which it is associated. These loops 
are short-circuited when the external gills disappear. 

In forms with internal gills, each aortic arch breaks up into 
capillaries in the demibranch and becomes divided into a ventral 
afferent branchial artery and a dorsal efferent branchial artery. 

In vertebrates with a pulmonary respiration, aortic arches I 
and II, in the mandibular and hyoid arches, respectively, dis- 
appear. Arch III, in the first branchial arch, persists as the 
connection between the internal and external carotid arteries, 

Internal External 
carotid carotid 





A B C D 

Fia. 143. Diagrams of aortic arches. A, hypothetical primitive type. B, in 
frog. C, in chick. D, in man. (After Kingsley.) 

while the dorsal aorta between arches III and IV disappears. 
Arch IV becomes the systemic arch connecting the dorsal and 
ventral aortae (Fig. MSB). In birds (Fig. 143C), the arch on the 
left side disappears; in mammals (Fig. 143D), that on the right 
degenerates. Arch V is greatly reduced and frequently disap- 
pears or has at most a vestigial and transitory existence. From 
arch VI there grow back to the lungs the pulmonary arteries. 
The portion of the sixth arch distal to the pulmonary arteries 
is reduced in caliber and is known as the ductus arteriosus. It 
becomes occluded and degenerates in all the amniotes except 
some few reptiles. 

Intersegmental arteries. From the dorsal aortae are given 
off small branches between the myotomes (Fig. 142B). Some of 
these intersegmental arteries persist as the intervertebral ar- 
teries. The more anterior ones becomes united on either side by 


a dorsal longitudinal vertebral artery. These vertebrals subse- 
quently fuse to form an anterior basilar artery which divides be- 
hind the pituitary, the two halves uniting with the internal 
carotid on either side. The posterior halves of the vertebral 
arteries fuse to form the spinal artery which runs back beneath 
the spinal cord. In the region where the anterior limb buds are 
developing, intersegmental arteries grow out, to give rise to the 
subclavian arteries. Similarly, in the region of the pelvic limb 
buds, intersegmental arteries give rise to the iliac arteries. In 
the amniota, the allantoic arteries grow out from the iliac arteries 
into the walls of the allantois. These become so important 
that for some time it appears as though the iliac arteries were 
derived from the allantois instead of the reverse. These allan- 
toic arteries, which degenerate at the time of birth, are known as 
the umbilical arteries in mammals as they traverse the umbilical 
cord and supply the placenta. 

Other important intersegmental arteries become the renal 
arteries of the kidneys and the genital arteries of the gonads. 

Mesenteric arteries. From the dorsal aorta, a number of 
ventral branches, originally paired, but soon fused to become 
median vessels, pass down the dorsal mesentery. They unite 
with the capillaries of the yolk sac which they supply with blood. 
Later, some of them develop branches over the alimentary canal 
which persist after the loss of the yolk sac as the coeliac and 
mesenteric arteries. 

The veins. There are two primitive venous systems: the 
somatic system, comprising the cardinal veins; and the splanchnic, 
including the vitelline (ornphalomesenteric) and, in amniotes, the 
allantoic (umbilical) veins. The cardinal veins are replaced by 
cayal veins; the vitelline veins become transformed into a 
hepatic-portal system. The allantoics disappear at hatching 
(or birth). Finally, there are the pulmonary veins. In general, 
the history of these transformations may be summed up in the 
statement that the primitive independent venous systems become 
transformed into a system wherein an accompanying vein is 
developed for every artery. 

The vitelline veins (Fig. 144). These vessels, and their con- 
tinuation, the subintestinal vein (in small-yolked forms), are the 
first vessels formed in the embryo. In the amniotes, two veins 



grow out from these into the wall of the allantois to become the 
allantoic veins of the sauropsida (umbilicals of mammals). In 
man, however, the umbilical veins actually appear before the 
vitelline veins. 

It has been noted previously that the vitelline veins pass 
around the liver on their way to the heart. As the liver enlarges, 
it surrounds the vitelline veins, and these become broken up in 
the liver tissue to form a great capillary network. In the am- 
niota, the allantoic (umbilical) veins are similarly absorbed. The 
proximal portions of the vitelline veins, from the liver to the sinus 
venosus, are now known as the hepatic veins; the distal portions 





FIG. 144. Diagrams to show three stages in the development of the hepatic-portal 
venous system, based on conditions in man. (After Hochstetter.) 

are called the portal veins. Of the umbilical veins, the right de- 
generates; the left for a time maintains a direct connection 
through the liver to the hepatic veins, known as the ductus 
venosus. This connection disappears at the time of birth. After 
the disappearance of the yolk, the portal vein and its tributaries, 
of which the most important is the mesenteric vein, carry blood 
from the digestive canal to the liver. 

The anterior cardinal veins. The original plan of the cardinal 
system is that of an H in which the upper limbs represent the 
anterior cardinals; the cross-bar the common cardinals, with 
the heart in the middle of the cross-bar; and the lower limbs 
represent the posterior cardinals (Fig. 145). The anterior cardi- 
nals arise as a drainage system for the blood passing into the head 
from the carotid arteries. 

The anterior cardinals are often called the internal jugular 



veins. From these, parallel veins, known as the external jugular 
veins, branch off in the ventral region of the head. Veins from 
the vertebral region (vertebral veins) and from the pectoral ap- 
pendages (subclavian veins) soon develop. In most vertebrates 
the common cardinals and the proximal portion of the anterior 
cardinals, i.e., up to the point where these tributary veins di- 
verge, persist as the precaval veins. In some mammals, a cross- 
connection is formed between the anterior cardinals, after which 
the portion of the left anterior cardinal, proximal to the anasto- 



Fio. 145. Diagrams to show three stages in the development of the caval venous 
system. Generalized (supra-cardinals omitted). 

mosis, and the left common cardinal become the coronary vein 
draining the wall of the heart. The corresponding vessels on 
the right side persist as the precaval (anterior caval) vein. 
' The posterior cardinals. Each posterior cardinal lies dorsal 
to the mesonephros which it drains. Beneath each mesonephros 
is developed a subcardinal vein. In the anamniotes these veins 
arise as tributaries of the posterior cardinals, returning blood from 
the tail where they are united to form the caudal vein. Later, 
they lose direct connection with the parent vessels and return 
blood from the tail region to the mesonephros as the renal-portal 
veins. The posterior portions of the subcardinals fuse as the 
interrenal vein, which acquires a secondary connection with the 


hepatic vein, and persists as the postcaval vein. In the am- 
niotes the postcaval vein is a complex which arises partly from 
the hepatic veins, partly from appropriated portions of the 
posterior cardinals and subcardinals, and partly from the supra- 
cardinals, a pair of vessels dorso-mesial to the posterior cardinals. 
It eventually replaces the posterior cardinals, so that the only 
blood vessels entering the right side of the heart are (1) the pre- 
caval vein returning blood from the head, pectoral region, and ap- 
pendages; and (2) the postcaval vein returning blood from the 
trunk and pelvic appendages as well as all blood from the digestive 
canal conveyed by way of the hepatic-portal system. 

The pulmonary veins. These enter the left atrium and are 
new vessels which grow backward from the heart to the developing 

The lymphatic system. This system serves to return to the 
veins the blood plasma which has escaped from the capillaries 
(Fig. 146). It contains white blood corpuscles of the ameboid 
type (lymphocytes) which have the power of making their way 
through the capillary walls. The lymphatics apparently origi- 
nate as intercellular spaces in mesenchyme which later become 
confluent and acquire a limiting endothelium. Like the blood 
vessels, the lymphatic capillaries anastomose and form larger ves- 
sels which drain into the veins. The walls of these central vessels 
are often muscular, and localized areas known as lymph hearts 
are found. So, too, localized distensible sacs, the lymph sacs, are 
not unknown. Some of these become lymph glands. The spleen, 
already alluded to in the section on mesenteries (page 198), is a 
hemolymph gland in which both lymphocytes and erythrocytes 
are proliferated. 

THE FROG (SEE ALSO CHAPTER XI). In the frog (Fig. 147), 

the primordia of the vitelline veins first appear and grow to- 
gether as a loose aggregate of cells in front of the liver. Around 
this the coelom grows in from right and left to form the peri- 
cardium. Meantime the primordium of the heart endocardium 
develops from the loose aggregate of cells referred to above. 
The inner wall of the coelom (splanchnic mesoderm) becomes the 
myocardium. The atrium is divided by an interatrial septum 
into two auricles, right and left. The ventricle remains a single 



Superficial lymphatics 

Jugular lymph sac 
Subclavian lymph sac 

Lymph gland 
Deep lymphatics 

Thoracic duct 
Retroperitoneal lymph sac 

Cisterna chyli 

Posterior lymph sac 
Superficial lymphatics Lymph gland 

FIG. 146. Reconstruction of primitive lymphatic vessels in human fetus of two 
months. (From Arey after Sabin.) 

Arteries External Aortic .. , . 

carotid ^^L_ V ^ ] Dorsal 
Internal I. I.f "I ivy VI .. aorta aorta 

carotid : 







cardinal cardma1 ' 

FIG. 147. Diagram of embryonic vascular system of early tadpole. (After 



Aortic loops develop in the external gills, corresponding to 
aortic arches III, IV, and V. After the appearance of the in- 
ternal gills, the ventral limb of the loop becomes the afferent 
branchial artery, while the dorsal limb becomes the efferent 
branchial artery. A similar differentiation takes place in arch VI. 
With the loss of branchial respiration, arch III becomes the proxi- 
mal portion of the carotid arteries, arch IV the systemic arch 
which persists on both sides, and arch V disappears, while from 
arch VI arise vessels which carry blood to both the lungs (pul- 
monary arteries, Fig. 143B) and skin (cutaneous arteries). 

The vitelline veins anterior to the liver fuse to become the 
hepatic vein: posterior to the liver, the right vitelline vein dis- 
appears, the left becomes the hepatic-portal vein. The anterior 
cardinal veins become the internal jugular veins; the common 
cardinals become the precaval veins. The posterior cardinal 
veins fuse between the mesonephroi, and a new vein grows back 
from the hepatic vein to the right posterior cardinal, to form the 
postcaval vein. The posterior cardinals, anterior to their junc- 
tion with the postcaval, degenerate. Posterior to this junction 
they persist as the renal-portal veins carrying blood from the 
iliac veins to the kidneys. 

THE CHICK (SEE ALSO CHAPTER xii). In the chick (Fig. 148), 
the endocardium of the heart arises as the forward extension of 
the vitelline veins, which soon fuse as the pericardia! primordia 
are brought together beneath the head. The myocardium is 
formed as in the4rog. The right and left halves of the heart 
^re completely separated by three septa: the septum aortico- 
puliAonale, which divides the bulbus into a chamber on the right 
leading to the pulmonary arteries and one to the left leading to 
the dorsal aorta; the interventricular septum, which divides the 
ventricle; and the interatrial septum, which divides the atrium 
into two auricles. This separation is completed at the end of 
the first week of incubation. The sinus venosus is incorporated 
in the right auricle. 

Six aortic arches are formed: I and II disappear on the third 
and fourth days of incubation; III forms the proximal portion 
of the internal carotid artery; IV disappears on the left side 
but persists as the systemic arch on the right; V disappears; the 
pulmonary arteries arise from VI, but the distal portion of the 



right arch remains as the ductus arteriosus until the chick hatches 
(Fig. 143C). 

The vitelline veins unite behind the sinus venosus to form the 
meatus venosus which later becomes the hepatic vein. The 
mesenteric vein becomes the portal vein, and the vitelline veins 
disappear at hatching. The allantoic veins grow backward from 
the common cardinals to join the capillaries of the allantois; 
the right allantoic degenerates on the fourth day, and the left 
acquires a new connection with the meatus venosus, by way of the 




FIG. 148. Diagram of embryonic vascular system of chick. (After Kingsley.) 

left hepatic vein. The allantoic vein degenerates at hatching. 
Two precaval veins are formed from the proximal portions of 
the anterior cardinals and common cardinals. The posterior 
caval vein arises from (1) a branch of the meatus venosus which 
grows back to meet the right subcardinal vein, (2) the fused sub- 
cardinals which carry blood from the mesonephros, and (3) the 
renal veins which develop in connection with the metanephros. 
The anterior ends of the posterior cardinals disappear, while the 
posterior ends supply the mesonephros and, after its degenera- 
tion, the common iliac veins, which pass directly to the post- 
caval vein. 

MAN (SEE ALSO CHAPTER xili). The heart arises in man 
(Fig. 149) much as in the chick; but the subsequent partition- 


ing of this organ into right and left halves is more complicated, 
for two atrial septa are formed. The ventricle is separated by an 
interventricular septum, and the bulbus is divided by two septa 
which unite to form the septum aortico-pulmonale. The sinus 
venosus is incorporated in the right atrium. 

The aortic arches are formed and have the same history as 
those of the chick, with the exception that it is the left fourth 
aortic arch which becomes the systemic arch (Fig. 143D). 

The anterior portion of the right vitelline vein becomes the 
hepatic vein; the hepatic-portal arises from the posterior portion 
of the vitelline veins anterior to their junction with the mesenteric 

Postcardinal veins Precardinal veins 

Vftelline artery^ [ / ^Descending aorta 

Umbilical arti 

^~^~^i^3&>" i // R\ //nn\i\ \ \ 

* A or tic arches i and 2 

Body stalk/ I/ \ ^^ J \ \ Heart 

Umbilical vein/ \^_ ^^ \ Sinus venosus 

Yitelline veins 
FIG. 149. Diagram of embryonic vascular system in man: (From Arey after Felix.) 

vein. The anterior cardinals are united by an anastomosis (left 
innominate vein), and the left common cardinal disappears with 
the exception of the coronary vein. The right common cardinal, 
together with that portion of the anterior cardinal as far as the 
branching of the left innominate, becomes the precaval vein. 
The postcaval vein is a complex formed from (1) a branch of 
the hepatic vein, (2) the anterior portion of the fused sub- 
cardinals, (3) part of the fused supracardinals, and (4) the fused 
posterior portion of the posterior cardinals. The anterior por- 
tions of the posterior cardinals separate from these veins, unite by 
means of an anastomosis, and drain into the right precaval vein. 
They are then known as the azygos (right) and hemiazygos (left) 
veins. Of the umbilical veins, the left only persists, with a 



direct connection through the liver by means of the ductus 
venosus. At birth this duct closes and the umbilical vein dis- 


The skeleton of vertebrates consists of a system of supporting 
and protecting elements developed from mesenchyme. These 
elements pass through several conditions in later development. 
The primordia of the skeletal elements are preformed in con- 
nective tissue. These become transformed into cartilage, a 
process known as chondrification, through the activities of spe- 
cialized cells, the chondrioblasts. Cartilage in turn is trans- 
formed into bone, through the action of osteoblasts, the process 
being known as ossification. Bones that pass through these three 
stages are known as cartilage bones. In the formation of some 
bones, the cartilaginous stage is omitted; these are known as 
membrane bones, j Both cartilage and bone are typically sur- 
rounded by a membrane of mesenchyme which is called the 
perichondrium or periosteum, as the case may be. The separate 
elements of the skeleton are connected with each other by liga- 
ments, by cartilage, or in a bony union. 






FIG. 150. Diagram to show the skeleton-forming regions as seen in the tail region 
of a vertebrate. (After Kingsley.) 

Skeletogenous regions. The principal regions where skeleton 
may be formed in the vertebrate body (Fig. 150) are (1) the 



Derm is 


dermis of the skin, (2) the median sagittal planes between the 
myotomes on the dorsal and ventral sides of the body, (3) the 
right and left frontal planes between the dorsal and ventral 
muscle masses, (4) the transverse planes between the myotomes, 

(5) around the notochord, neural tube, and axial blood vessels, 

(6) in the visceral arches, and (7) in the paired appendages. 
Skeletal elements formed in (1) are called the dermal skeleton; 
those formed in (2) to (5), the axial skeleton; those formed in (6), 
the visceral skeleton; and those formed in (7), the appendicular 
skeleton. The skull contains elements from all but the appen- 
dicular skeleton. 

The dermal skeleton. Among living vertebrates the most 
primitive example of derm bones are the placoid scales (Fig. 151) 

of the cartilage fish which are 
formed in exactly the same 
way as teeth (Chapter VIII). 
In the dermal skeleton two 
types of bones are distin- 
guished. The investing bones 
(dermal plates) serve to en- 
FIG. 151. Section of developing placoid velop regions of the head 

scale (Squalus acanthias) to show origin of and trunk. The substituting 
primitive dermal bone. Compare Fig. boneg become so clogel allied 

119. (After Rmgsley.) -,i ,u A -i / 

with the cartilaginous bones as 

to become fused with them or even to replace them in ontogeny. 
Many of the cranial bones are of this type. They may be dis- 
tinguished by the fact that they 
do not pass through a cartilagi- 
nous stage in development. 

The axial skeleton. The 
primitive axial skeleton is the 
notochord, whose origin has 
been discussed in Chapter V. 
Around this a connective tissue Fia 152 - ~~ Section through sclerotome 

,,,./. T , , of lizard (Scleporus) to show arcualia. 

sheath is formed by mesenchy- (After Kingslev .) 

mal cells. The mesenchyme 

from each sclerotome now forms four little blocks, the arcualia 

(Fig. 152), two dorsal to the notochord and two ventral, from 

which the arches and centra of the vertebrae are formed, as well 







as the primordia of the ribs. The posterior arcualia of each so- 
mite unite with the anterior arcualia of the succeeding myotome 
to form the definitive vertebra, which thus comes to lie at the 
point of separation between two myotomes. Eight elements are 
thus concerned with a single vertebra: right and left dorsal 
arcualia from the anterior half sclerotome, and from the posterior 
half sclerotome, and the corresponding ventral elements. 

The vertebrae. In the prevertebral masses so formed appear 
centers of chondrification, one on each side of the spinal cord 
arid one or more below the cord. These form, respectively, the 
neural arch and the centrum of the vertebrae (Fig. 153). In the 
tail region, two centers of chondrification arise below the centrum, 

FIG. 153. Section to show ossification centers in human vertebra and ribs. (After 


enclosing the caudal prolongation of the dorsal aorta, and form a 
heinal arch. With the chondrification of the vertebrae the noto- 
chord disappears in all but the most primitive vertebrates, per- 
sisting only between the vertebrae as nuclei pulposi of the inter- 
vertebral discs. Finally the vertebrae become ossified, and the 
spines, zygapophyses, and other differentiations are developed. 

The ribs. Except in the caudal region, lateral processes 
arise from the vertebral primordia and grow out into the myo- 
septa. They later become cartilaginous, and finally true bone. 
These are the ribs, of which there are two types, dorsal and ven- 
tral, distinguished according to the part of the vertebra from which 
they originate. 

The sternum. The sternum, or breast bone, arises in the 
amphibians from the coalescence of two longitudinal bars of 
cartilage, which later articulate with the coracoids of the pectoral 
girdle, but do not come in contact with the ribs. In the amniota, 



* Mesosterna 



the sternum arises from the fusion of the ventral ends of the an- 
terior rib rudiments. In this way there arise two longitudinal 

bars, from which the unpaired sternum 
is formed by fusion along the mesial 
line (Fig. 154). 

The skull. The skull is a complex 
of skeletal elements, arising from the 
chondrocranium, or primitive cranium 
of cartilage bones, which is derived in 
part from the protective covering of 
the brain and sense organs (neuro- 
cranium), and in part from the sup- 
porting elements of the visceral arches 

FIG. 154. - Diagram to show on- (gplanchnocranium). This is supple- 
mented by numerous investing and 
substituting bones from the original 
dermal skeleton (dermocranium). 

Neurocranium. The neurocranium arises from the head 
mesenchyme which, as has been said, cannot be traced to any 
definite somites. In this mass, which completely invests the 
brain and sense organs, definite centers of chondrification appear. 
These masses unite to form the chondrocranium of the cartilage 
fish (Fig. 155). If the notochord be used as a point of orientation, 

gin of mammalian 
(After Kingslcy.) 




Otic capsule 



Visceral arches 

FIG. 155. Diagram showing components of chondrocranium (Squalus acanthias). 

(After Kingsley.) 

on either side of it is found a parachordal bar. In front of each 
of these is a separate rod; these are the trabeculae. Between 
the two parachordals and around the notochord, the basilar plate 
arises as the support of the epichordal brain. The trabeculae 
also fuse in front, to form the ethmoid plate which supports 


the prechordal brain, but remain separate at their posterior 
ends to form an opening through which the pituitary projects 
downward. In front of the ethmoid plate the trabeculae grow 
forward as the cornua. Dorsal to each trabecula, another longi- 
tudinal bar, the sphenolateral, arises. Between these two bars 
the cranial nerves make their way to the exterior. 

Around each of the major sense organs a cartilaginous capsule 
develops. The olfactory capsules unite with the cornua, eth- 
moid, and sphenolaterals. The optic capsule rarely develops 
fully, usually persisting in the connective tissue stage as the 
sclera of the eyeball. The otic capsule, however, becomes com- 
pletely chondrified and unites with the parachordals and the 
latero-sphenoids. Between the two otic capsules and spheno- 
laterals arises a dorsal plate which forms a roof for the brain. 
In the amniotes, one or more neck vertebrae are consolidated with 
the occipital region. 

The splanchnocranium. The digestive canal in the head 
region consists of the mouth, oral cavity, and pharynx, the walls 
of the pharynx being penetrated by the visceral clefts. As there 
is no coelom in this region, the lateral mesoderm is not divided but 
gives rise to mesenchyme which foreshadows the cartilaginous 
bars supporting the wall of this part of the body. These visceral 
arches are the mandibular, hyoid, arid four (or more) branchial 
arches. The mandibular arch divides into dorsal and ventral 
portions, of which the dorsal portion becomes the pterygoquadrate 
cartilage (upper jaw of cartilage fish) while the ventral portion 
becomes the meckelian cartilage (lower jaw). The hyoid arch 
divides into a dorsal hyomandibular cartilage, and a ventral hyoid 
cartilage which is usually divided into several centers of chondri- 
fication. The hyomandibular acts as a suspensory element for 
the jaws in the fish. It is homologized with a bone of the middle 
ear, the columella, in amphibians, and the stapes of mammals 
(see page 269). The hyoid gives rise to the support of the 
tongue. The branchial arches are usually divided into four parts 
and act as gill supports in the anamniota and disappear or become 
laryngeal cartilages in amniota. 

Ossification of the chondrocranium. The limits of this text 
will not permit of an enumeration of all the bones formed from 
the chondrocranium (Figs. 156, 157, 158). They may be grouped, 



however, as follows: (1) the occipitals, formed from the occipital 
vertebrae; (2) the sphenoids, arising from the parachordals, 
basilar plate, trabeculae, and latero-sphenoids ; (3) the ethmoids, 





s Basioccipital 

FIG. 156. Diagram showing components of vertebrate skull, generalized. Ventral 
view. Chondrocranium stippled, dermal elements in outline. (After Kingsley.) 

from the ethmoid plate and nasal capsule; (4) the otics, from the 
otic capsule. The pterygoquadrate bar gives rise to the ptery- 
goid bones and the quadrate (which in mammals becomes the 
incus of the middle ear). The meckelian cartilage gives rise to the 





^ Postorbital 




FIG. 157. Dorsal view of skull diagrammed in Fig. 156. 

articular bone at its distal extremity. This becomes the malleus, 
another ear-bone, of the mammals. The remainder of the 
meckelian persists as cartilage. In the hyoids and the branchials, 
bones are formed which retain the names of their cartilaginous 



The dermocranium (Figs. 156, 157, 158). The derm bones 
which invest and, to some extent, supplant the elements of the 
chondrocranium are too numer- 
ous to be more than mentioned 
here. The dorsal derm bones 
are, from front to rear, the nasals, 
frontals, and parietals, together 
with a number of smaller bones 
which appear in variable quantity 
in the different classes. The 
principal lateral elements, from 
front to rear, are the premaxillae, 

Premaxilla j 








FIG. 158. Lateral view of skull dia- 
grammed in Figs. 1,56, 157. 

maxillae, jugals, quadratojugals, and 
squamosals. The floor of the chondro- 
cranium is invested by the parasphenoids, 
palatines, and vomer. The lower jaw is 
invested by a series of bones of which the 
most important is the dentary. 

The appendicular skeleton. The sim- 
plest forms of appendages, the unpaired 
and paired fins of fish, contain a skeleton 
consisting of parallel cartilaginous rods, 
which are divided into proximal portions, 
basalia, embedded in the body, the distal 
portions, radialia, extending into the free 
appendages. The paired appendages of 
fish are paddle-like fins ; in tetrapods they 
are jointed legs. In both, the skeleton is 
divided into a basal girdle and a free ap- 
FIG. 159. Diagram of ap- pendicular skeleton (Fig. 159). 
pendicuiar skeleton, tetra- ^he girdles. The girdles are in the 

pod type, showing homol- f f . ,, , PI-IJV 

ogies of pectoral elements form f inverted arches, of which the 

above and to left; pelvic pectoral girdle is united to the axial skel- 
elements below and to e t O n in fish and free in the tetrapods, 

right. (After Kingsley.) whHe ^ ^ ^^ ^^ free ^ 

fish, is united to the axial skeleton in the tetrapods. Each arch 



typically consists of three portions. The dorsal one in the pec- 
toral girdle is the scapula; in the pelvic girdle it is called the 
ilium. The two ventral elements of the pectoral girdle are the 
precoracoid (anterior) and the coracoid (posterior), while the 
corresponding elements of the pelvic girdle are the pubis and 
ischium. In the shoulder region, the clavicle, a derm bone, 
becomes associated with the pectoral girdle. 

The free appendages. The pectoral and pelvic appendages 
are very similar. Each has three segments: proximal, inter- 
mediate, and distal. The proximal segment of the pectoral ap- 
pendage contains one bone, the humerus, while the corresponding 
bone of the pelvic limb is called the femur. The intermediate 
portion of the pectoral limb possesses two bones, the radius and 
ulna; while the corresponding bones of the pelvic appendage are 
the tibia and fibula. The distal segment is divided into three 
regions of which the proximal portion contains nine or ten bones, 
the carpalia of the pectoral appendage, tarsalia of the pelvic. 
The intermediate portion contains five metacarpalia or meta- 
tarsalia, respectively. The distal portion contains the free 
phalanges of the fingers or toes. 










Free appendage 


Phalanges (I-V) 

Phalanges (I-V) 

Origin of the appendicular skeleton. All the bones of the 
appendicular skeleton, with the exception of the clavicle, are 
formed from a mesenchymal blastema in the limb buds by the 
appearance of centers of chondrification. The origin of this 


mesenchyme is probably from the somites, but the details of the 
process are still imperfectly understood. 

THE FROG. 1 Nine vertebrae are formed, of which the first is known as the 
cervical vertebra, or atlas, the succeeding seven are the abdominal vertebrae, and 
the last is called the sacral vertebra as it is to this that the pelvic girdle is attached. 
No caudal vertebrae are formed, but three strips of cartilage enclose the notochord 
and form the primordium of the adult urostyle. Dorsal ribs are differentiated, but 
these remain rudimentary and fuse with the transverse processes of the vertebrae. 
The sternum arises from the fusion of two longitudinal bars of cartilage which never 
attain connection with the ribs. It persists anterior and posterior to the pectoral 

The cartilage bones of the skull are the exoccipitals, prootics, stapes, ethmoids, 
and the pterygoquadrate (in part), articularc, mentomeckelian, hyoid, and branchials. 
The derm bones are the fronto-parietal, nasals, prcrnaxillae, maxillae, quadratojugals, 
squamosals, parasphenoid, palatines, vomers, and dentaries. 

In the pectoral girdle develop the scapula, coracoid, and precoracoid, the last of 
which is replaced by the clavicle. In the pelvic girdle only the ilium and ischium 
ossify. Only four digits are present in the hand, the thumb (pollex) being absent. 

THE CHICK. There are sixteen cervical vertebrae, of which the first is the atlas, 
and the second, which has appropriated the centrum of the first, is the axis; five 
thoracic vertebrae; about six lumbar vertebrae; two sacrals; and about fifteen 
caudals. The last thoracic, all lumbars, and sacrals and five caudals are fused to 
the pelvic girdle. The last four caudals are fused into a pygostyle. Dorsal ribs are 
formed by the cervical and the thoracic vertebrae. The sternum arises from two 
longitudinal bars of cartilage which unite in the median line. It is distinguished by 
the development of a large keel (carina) for the attachment of the pectoral muscles. 

The cartilage bones of the skull arc the basioccipital, exoccipitals and supra- 
occipitals; prootics, epiotics, and opisthotics; basisphenoid, orbitosphcnoids, and 
alisphenoids; the ethmoid; quadrate, articular, meckelian cartilage; stapes, hyoid, 
and branchials. The derm bones are the frontals, parietals, nasals, lachrymals, pre- 
maxillae, maxillae, jugals, quadratojugals, squamosals, pterygoids, palatines, para- 
sphenoids, vomer, angular, supra-angular, opcrcular, and dentary. 

The pectoral girdle develops a scapula and coracoid, together with a dermal 
clavicle. Ilium, ischium, and pubis ossify separately in the pelvic girdle. Five 
digits are performed in the pectoral appendage; of these the first and fifth fail to 
develop further. Five also appear in the embryonic skeleton of the pelvic appendage; 
the fifth soon disappears, and the first is extremely short and develops no phalanges. 

MAN. Seven cervical vertebrae, including the axis and atlas, twelve thoracic, 
five lumbar, five sacral, and four caudal vertebrae are formed. Of these, the sacral 
vertebrae are united to the pelvis, and the caudal vertebrae are frequently fused to 
form the coccyx. Primordia of ribs are formed by all vertebrae except those follow- 
ing the first caudal. Only the thoracic segments, however, develop complete ribs. 
The sternum arises from two longitudinal primordia with which the first eight or 
nine ribs acquire cartilaginous connections. 

The cartilage bones of the skull are the occipital (in part), the sphenoid, the 
ethmoid, the turbinates, temporals (in part), the stapes, malleus, incus, and hyoid. 
The malleus and incus are the representatives of the articular and quadrate. The 

1 The details of the skeleton in this and succeeding paragraphs are for reference 



derm bones arc the occipital (in part), temporals (in part), frontal, parictals, lachry- 
mals, nasals, vomer, maxillae, zygomatics, palatines, and mandible, the last-named 
bone representing the fused dentaries. It is apparent that many of the bones of the 
human skull are the result of the fusion of separate centers of ossification which 
represent skull elements of the lower vertebrates. The second and third visceral 
arches contribute to the formation of the hyoid, the others to the laryngcal cartilage. 
The pectoral girdle consists of the scapula, with which is fused the coracoid. 
There is no precoracoid, but a dermal clavicle is present. The centers of ossification 
that represent the pubis, ischium, and ilium fuse to form an innominate bone. The 
free appendages terminate in five digits. In conclusion, it should be mentioned 
that the adult condition of the human skeleton is not attained until the age of 


The musculature of the vertebrate is derived from mesen- 
chymc (Fig. 160), of which the greater part originates from the 
myotornes and gives rise to striated muscle cells, controlled by the 
central nervous system, the skeletal musculature. A portion, 

Neural tube 




muscle mass 

muscle mass 


FIG. 160. Diagram of transverse section through vertebrate embryo in region of 
limb bud, to show origin of appendicular muscles. (After Corning.) 

however, originates from splanchnic mesoderm and gives rise to 
non-striated (smooth) muscle cells(found in the skin, surrounding 
the alimentary canal, blood vessels, and the urogenital organs. 
They areJ controlled by the autonomic nervous system (page 254), 
and make up the visceral musculature. Several exceptions to 
these general statements should be noted. The muscle cells of 
the heart are striated; the muscles derived from the visceral 
arches are both) striated and controlled by the central nervous 



system, although derived from lateral mesoderm. It will be 
noted later that the muscles of the iris of the eye (page 266) and 
of the sweat glands (page 246) are apparently ectodermal in 

Dermal musculature. In the skin are found striped muscles 
which are derived from skeletal musculature (see below) but 
which have lost their attachment to the skeleton. The dermal 
musculature is best developed in the amniotes. The muscles of 
expression in man are dermal muscles supplied by the seventh 
cranial nerve (see Chapter X). 

Axial musculature. In this section are included all the 
muscles arising from the myotomes and attached to parts of the 
axial skeleton, which they move. They are originally meta- 
meric, but their later history is obscured by subsequent migra- 
tion, fusion, splitting, budding, and degeneration. The inter- 
costals, between the ribs, however, preserve their original metam- 
erism, which in the others may be traced to some extent by the 
innervation, since the connection between a spinal nerve and 
the muscle mass it supplies is established early in orgariogeny 
and remains constant. Thus it can be shown that the muscu- 
lature of the diaphragm, 
supplied by the phrenic 
nerve, arises from a cervi- 
cal my o tome. 

Cranial muscles. 
Like the cranium, the as- 
sociated muscles are de- 
rived from different 
sources and consist of 
skeletal and visceral 
muscles. The muscles 

of the eyeball arise from FIG. 161. Head of embryo dogfish (Squalus 
the three preotic myo- M\ Rowing preotic somites (A, B C) 

._. ^ . . - and cranial nerves (V, VII, IX, X). (After 

tomes (Fig. 161), of which Kingsley.) 

the first supplies all the 

muscles of the eyeball except the superior oblique, derived from 
the second myotome, and the lateral rectus, supplied by the third 
head myotome. These are innervated by the third, fourth, and 
sixth cranial nerves, respectively. The tongue musculature is 



derived from the myotomes associated with the occipital vertebrae 
and supplied by the twelfth cranial nerve. The muscles of masti- 
cation, the facial muscle, and the laryngeal muscles, together with 
those of the ear bones, arise from the visceral arches (Fig. 162), 


Glossopharyngeal t Facial 


Branchial arches 



FIG. 162. Diagram to show primitive visceral muscles in relation to visceral skele- 
ton and cranial nerves. (Hypothetical after Wilder.) 

and are supplied by cranial nerves V, VII, IX, X, and XI (see 
Chapter X). 

Appendicular muscles. In the anamniotes, these muscles 
arise from the myotomes; among the amniotes, their origin is 
doubtful, as the limb bud develops as an undifferentiated mass of 
mesenchyme surrounded by ectoderm. In this blastemal mass, 

Dorso - medial 
muscle primordis 


Ventro - lateral 
muscle primordia 

FIG. 163. Reconstruction of the pectoral muscle masses in a 17-mm. Necturus; 
(Prepared by H. F. DeBruirie.) 

muscles and bones are laid down, the differentiation proceeding 
from the proximal toward the distal end. The pectoral muscles 
differentiate before those of the pelvic appendage. The appen- 


dicular muscles are found in antagonistic groups: protractors, 
which move the limb forward; and retractors, which have the 
opposite effect; levators, which raise the limb; and depressors, 
which contract in the opposite direction. Like the axial muscles, 
these have become highly modified and specialized among the 
tetrapods (Fig. 163). 

Visceral muscles. Under this head are included the muscles 
lining the alimentary tract, lungs, vascular organs, and uro- 
genital system. All arise in the mesoderm which surrounds the 
endothelial lining of the organs concerned. The muscle cells of 
the heart arise as smooth muscle cells which become striated in 
later development. It is interesting in this connection that the 
smooth muscle cells of the bladder of the dog have been trans- 
formed into what are apparently striate muscles when this organ 
is made to pulsate rhythmically by continued irrigation. 


The following structures are derived from the middle germ 

A. The notochord 

B. The mesoderm 

I. The lateral mesoderm 
Epithelium of the coelom 

Pericardial cavity 
Pleural cavity 
Peritoneal cavity 

Dorsal mesentery 
Ventral mesentery 

II. The intermediate mesoderm 







Genital ducts 

Wolffian (mesonephric) duct 
Miillerian (oviducal) duct 
External genitalia (also ectodermal) 
Adrenal glands 


(Suprarenals from ectoderm) 

C. The mesenchyme 

III. (Principally from splanchnic mesoderm) 

The blood corpuscles 
Blood plasma 
Blood vessels 



The lymphatics 

IV. (Principally from the axial mesoderm) 

Connective tissue 




Splanchnocranium (or visceral 






Visceral (from splanchnic mesoderm) 



Allen, E. 1932. Sex and Internal Secretion. 

Arey, L. B. 1934. Developmental Anatomy, 3rd Ed., Chaps. 9-13. 

Brachet, A. 1921. Traite d'embryologie des verte'bre's, Part II, Bk. 1, Chap. 2; 

Bk. 2, Chaps. 1-4. 

Hertwig, O. 1906. Handbuch, Vol. 1, Chap. 5; Vol. 3, Chaps. 1-7. 
Jenkinson, J. W. 1913. Vertebrate Embryology, Chap. 7. 

Keibel and Mall. 1910-1912. Human Embryology, Chaps. 11-13, 15, 18, and 19. 
Kellicott, W. E. 1913. Chordate Development. 
Kerr, J. G. 1919. Textbook of Embryology, Chaps. 4-6. 
Kingsley, J. S. 1926. Comparative Anatomy of Vertebrates, 3rd Ed. 

1925. The Vertebrate Skeleton. 

Lillie, F. R. 1919. The Development of the Chick, 2nd Ed. 
McMurrich, J. P. 1923. The Development of the Human Body, 7th Ed. 
Vialleton, L. 1924. Membres et ccintures des vertebres tetrapodes. 



The ectoderm, being the external germ layer, gives rise to the 
outer layer of the skin, the epidermis, which continues into all 
the openings of the bodyj Of these, the development of the 
mouth, the cloaca and its derivatives, and the visceral clefts 
has been discussed. There remain for consideration the open- 
ings of the nostrils, the chamber of the eye, and the external 
auditory meatus. These will be taken up in connection with the 
sense organs, which, together with the nervous system, form in 
development a sensory-nervous complex. 


The integument consists of two parts, the ectodermal epi- 
dermis, and the mesodermal dermis. The epidermis soon de- 


Basal cells 


Stratum intermedium 
Stratum germinativum 


^^^tr" 1?~ r *^*yi 

FIG. 164. Sections through human fetal skin; A, neck. B, chin. (From Arey 

after Prentiss.) 

laminates into two layers, the deeper germinativum, from which 
new strata are proliferated towards the exterior, and an outer 
periderm or embryonic skin (Fig. 164). Beneath the periderm, 




the outer cells of the germinativum are transformed into a horny 
layer, the corneum. The underlying dermis is essentially a 
supporting layer of mesenchyme cells derived largely from the 
outer side of the myotome, a region which is sometimes known 
as the dermatome. In the dermis are formed blood vessels, con- 
nective tissue,nbone, and muscle. The bony scales of fish are 
dermal in origin. 

Derivatives of the corneum. In the amniotes the horny layer 
of the epidermis is frequently fragmented to form horny scales 





FIG. 165. Diagrams showing similar development in A, scale; B, feather; and C, 

hair. (After Kingsley.) 

(Fig. 165 A), such as those of reptiles, or those found on the legs 
of birds, or the tails of rats, etc. Among the birds, scales are 
largely replaced by feathers which originate in much the same 


Unguis u . 
Subunguis_ _J Subungui 

FIG. 166. Diagrams to show ectodermal primordia of A, nail; B, claw; and C, hoof. 
Above in sagittal sections; below ventral view. (After Kingsley.) 

manner as scales. The epidermal plate, however, grows down 
like a cup to enclose a core of dermal origin (Fig. 165B). The 
epidermal sheath gives rise to the quill and barbs, while the core 
gives rise to the pulp, by means of which nutriment is supplied 
to the developing feather. Among the mammals, hair arises in a 





very similar fashion. An epidermal plate grows down into the 
dermis to form the hair bulb, the proximal end of which invagi- 
nates to receive a mesodermal core, the hair papilla, while 
around the whole is a mesodermal hair sheath (Fig. 165C). The 
hair papilla, however, does not grow out into the center of the 
hair as does the pulp of the feather. Claws, nails, and hoofs 
arise from the union of two epidermal primordia like those of 
scales, a dorsal unguis and a ventral subunguis (Fig. 166). 

Derivatives of the germinativum. The germiriativum, in ad- 
dition to producing the more superficial layers of the epidermis, 

gives rise to the glands of the skin 
(Fig. 167). Among the anamni- 
otes, these glands are usually uni- 
cellular and produce the mucus 
which serves to diminish the fric- 
tion of the skin against the water 
while swimming. Unicellular 
glands frequently aggregate to 
produce multicellular glands, such 
as the flask glands and cement 
glands of the anarnniotes, or the 
sebaceous (oil) and sudoriparous 
(sweat) glands of the mammals. 
The mammary glands of mammals are modified sudoriparous 
glands secreting the milk by which the new born are nourished 
through infancy. 

Derivatives of the dermis. Two types of pigmentation are 
to be distinguished in the integument. The first is produced by 
pigment secreted in the ectodermal epidermis, i.e., the melanin, 
of the frog tadpole. The second is produced by chromatophores, 
which are mesenchyme cells of the dermis. These secrete pig- 
ment granules and move toward the light to form a layer immedi- 
ately below the epidermis, some even wandering into the epidermis 

THE FROG. The ectoderm of the frog embryo is ciliated at 
6-mm. body length and remains so until the length of 20 mm. is 
attained, when the cilia disappear except on the tail which re- 
mains ciliated until metamorphosis. The jaws and oral combs 
of the tadpole are derivatives of the corneum and consist of rows 


FIG. 167. Section of Protopterus skin 
to show glands. (After Kingsley.) 


of horny denticles forming replacement series. The oral gland, 
or sucker, is a multicellular mucous gland derived from the 
germinativum and elevated by the elongation of its gland cells. 
It arises as a crescentic groove posterior and ventral to the point 
where the stomodeum will appear, then becomes V-shaped, and 
finally divides by the degeneration of the middle portion. The 
cement gland atrophies soon after the opening of the mouth. The 
pigmentation of the skin is derived from two sources, the melanin 
of the egg which is distributed to the epidermis, and the mesen- 
chymal chrornatophores (Fig. 199) which develop in the dermis. 

THE CHICK. The scales on the legs are typical reptilian scales 
and are derived from the corneum ; they sometimes bear feathers 
in the young* bird and so form a transition between scales and the 
characteristic avian feathers. The claws arise in the corneum 
from two primordia, a dorsal " claw-plate " and a softer " claw- 
sole." To prevent the sharp claws tearing the embryonic mem- 
branes, the concavity of the claw is filled with a pad known as the 
neonychium, derived from the corneum, which is lost after hatch- 
ing. The beak arises from the corneum around the upper and 
lower margins of the jaws. The egg tooth is a horny prominence 
on the dorsal side of the upper jaw, appearing on the sixth day of 
incubation but not taking on its ultimate shape until the four- 
teenth. It serves to aid in breaking the shell and is lost after 

MAN. The nails arise from nail-plates and sole-plates, of 
which the latter are rudimentary structures. They are covered 
during fetal life by the eponychium, consisting of the periderm 
and outer layers of the corneum. The hairs are arranged in 
patterns which have been* interpreted as reminiscences of the 
ancestral scales. The first growth of hair is called the lanugo; 
it is cast off, except over the face, soon after birth. The mam- 
mary glands arise from two longitudinal thickenings of the 
epidermis, known as the milk ridge. In later development the 
gland resembles an aggregation of sudoriparous glands. 


Although the nervous system and sense organs arise together 
and remain in functional continuity, it has become customary to 
distinguish the sense organs (receptors) from the nerves (trans- 


mittors) by which stimuli are passed on to the muscles or glands 
(effectors). ) Both the nervous system and the sense organs arise 
from specialized regions of the dorsal ectoderm, known respec- 
tively as the neural plate and the sense plates (placodes)l These 
represent an inward growth from the gerrninativum as opposed 
to the outward growth which produces the epidermis. In the 
frog this division is clearly indicated by a line of cleavage be- 
tween the outer epidermal ectoderm and the inner nervous ecto- 
derm. Both the neural plate and the sensory placodes with- 
draw from the surface and become subepidermal by a process of 
invagination. In this connection it is interesting to note that 
the optic placode is incorporated and invaginates with the neural 
plate so that when the retina of the eye develops, it does so from 
the brain. / 

The neural tube. 'The neural plate is an elongate structure, 
extending from the blastopore to the head region.! Local growth 
results in the incurving of this plate to produce a neural groove 
with conspicuous lips, the neural folds. As this growth con- 
tinues the groove sinks inward and the lips meet above it, thus 
converting the groove into a neural tube, which breaks away from 
the overlying epidermis and sinks into the interior. The cells 
at the margin of the neural plate form, at each dorso-lateral 
angle of the neural tube, a bar known as the neural crest, which 
subsequently segments into the ganglia. 

The neurons. The inner lining of the neural tube, corre- 
sponding to the outer layer of the neural plate, is called the 
ependyma. This is the center of cell proliferation (Fig. 170). 
Two types of cells are formed: the supporting cells, or spongio- 
blasts; and the embryonic nerve cells, or neuroblasts. The 
neuroblasts migrate out of the ependyma and form an inter- 
mediate mantle layer in which the}^ become transformed into 
neurons. These nerve elements have a prolongation at one 
end known as the axon or nerve fiber, while at the other are 
branched projections called dendrites. The axons grow but 
from the mantle layer into the outer layer of the cord, known as 
the marginal layer, where they secrete the medullary sheaths 
which act as insulating coats. Not all axons become medullated. 
Similar changes take place in the ganglia, whereby neurons and 
supporting cells are differentiated. 



Types of neurons. We may distinguish four types of neurons 
(Fig. 168), as follows: (1) Afferent neurons arising in the ganglia 
and sending their axons 
to the dorsal region of 
the neural tube. These 
convey excitations 
from the sensory recep- 
tors to the neural tube. 
Two sub-types are dis- 
tinguished : (a) the so- 
matic sensory neurons, 
conveying excitations 
from the exterior; and 
(6) splanchnic sensory 
neurons, conveying ex- 
citations from the vis- 
cera. (2) Efferent 
neurons, with their 
bodies in the ventral 
region of the neural 
tube, sending their 
axons to effectors 
(muscles or glands). 
Two sub-types are rec- 
ognized : (a) somatic 
motor and (6) splanch- 
nic motor. These af- 
ferent and efferent neu- 
rons form the periph- 
eral nervous system. 
(3) The intersegmental 
neurons have their bodies in the ventral portion of the neural tube 
and their axons are usually directed towards its posterior end. 
They serve to connect efferent neurons in the different segments 
of the body. (4) The suprasegmental neurons have their bodies 
usually in the dorsal portion of the neural tube and their axons are 
directed toward the anterior end of the tube, i.e., the brain. 
They serve to convey afferent excitations toward the brain and 
in that organ give rise to the great brain centers. The axons of 


FIG. 168. Diagram to show cross-sections of the 
spinal cord at three levels, the posterior level above. 
The dotted lines indicate the paths of neurons whose 
bodies lie wholly within the cord, suprasegmental 
to the left. 


these last two types of neurons form the descending and ascend- 
ing bundles of the brain and cord. 

The spinal cord. I The spinal cord, or neural tube exclusive 
of the brain, retains its primitive characteristics^ < The cavity, 
or neurocoel, persists as the central canal. (^Between each pair 
of vertebrae/ the afferent and efferent neurons) form a pair of 
spinal nerves which run out into the myotomes and hence have 
a metamerism equivalent to that of the myotomes, an important 
point in considering the homologies of the muscles. L In the region 
of the pectoral and pelvic appendages,) several of the segmental 
nerves combine to form the brachial and the sacral plexus, respec- 
tively. The cord becomes surrounded by an envelope of mesen- 
chyme known as the meninx, which in the higher vertebrates 
becomes divided into an inner pia mater and an outer dura mater. 
THe development of the nerves will be taken up in a later section. 

The brain. Whereas the cord is largely composed of afferent, 
efferent, and intersegmental neurons, by which certain reflex 
actions are directed, I the anterior end of the neural tube enlarges 
and differentiates inio the complex brain (Fig. 169). Here arise 
several centers in which the impulses received mainly from the 
major sense organs, nose, eye, and ear, are correlated. The brain 
may be divided into two major regions: the archencephalon, or 
prechordal brain; and the deutencephalon, or epichordal brain. 
With continued local growth, the archencephalon grows down in 
front of the notochord, thus forming the first or cranial flexure. 
At the same time, three dilations appear: the prosencephalon 
from the archencephalon; the mesencephalon at the point of the 
flexure; and the rhombencephalon from the deutencephalon.) 
It is convenient to associate the future history of the prosen- 
cephalon with that of the nose, the mesencephalon with that of 
the eye, and the rhombencephalon with that of the ear. 

The prosencephalon. The later history of the prosencephalon 
is complicated by the fact that|the optic placode is included in 
the neural tube at this point. Accordingly, we find^ihe prosen- 
cephalon dividing into an anterior telencephalon and a posterior 

The telencephalon. The anterior part of the telencephalon 
becomes the olfactory lobe, which receives the afferent neurons 
from the nose. From the roof develops the cerebrum, yhich be- 



comes the most complex and important center of associatioli! 
From the floor arises the optic part of the hypothalamus.^ There 
are two cavities, or telocoels (also known as the lateral ventricles). 







FIG. 169. Diagrams to show early development of the vertebrate brain in sagittal 
sections. A, prechordal and epichordal divisions. B, primary brain vesicles. 
C, definitive vesicles. The longitudinal broken line indicates division between roof 
and floor regions. (After von Kuppfer.) 

The diencephalon. The roof of the diencephalon gives rise 
to the thalamus in front, and the metathalamus behind; from 
the latter springs a dorsal diverticulum, the epithalamus. This 
structure, often known as the epiphysis, gives rise to something 
very much resembling an unpaired eye in early embryonic life; 
this later becomes the pineal gland of the adult, one of the so- 


called endocrine glands. /The eyes take their origin from the side 
of the diencephalon. The floor of the diencephalon gives rise to a 
ventral di verticulum the infundibular^ which grows downward 
to meet the advancing hypophysis from the stomodeum (see page 
181). The two later fuse to form the pituitary gland Janother of 
the endocrine series. Behind the irifundibulum, the noor of the 
diencephalon forms the mammillary part of the hypothalamus. 
It is evident that the thalamencephalon, often used as a synonym 
of the diencephalon, differs from it by the inclusion of the optic 
part of the hypothalamus, which is derived from the telencephalon 
although indistinguishable from the mammillary part of the 
hypothalamus in the adult. The thalami contain nuclei (masses 
of neurons) which receive afferent impulses from the optic, 
general sensory, and acoustic organs, and transmit impulses to 
and from the other centers of the brain. The cavity of the 
diencephalon persists as the diacoel (third ventricle). 

The mesencephalon. The roof of the mesencephalon gives 
rise to the corpora bigemina (quadrigemina in mammals), or optic 
lobes, the centers which receive afferent impulses from the eyes 
transmitted through the diencephalon.! The floor of the mesen- 
cephalon is the anterior portion of the brain stem, from which the 
motor neurons of the cranial nerves depart. The third and fourth 
cranial nerves originate from the mesencephalon. Its cavity is 
the mesocoel (or aqueduct).^ 

The rhombencephalon. "The hind-brain, like the fore-brain, 
is divided into two regions, metencephalon and myelencephalon, 
respectively. ~ 

The metencephalon. The roof of the metencephalon gives 
rise to the cerebellum, the center associated with hearingxexcept 
in mammals), the lateral line organs of anamniotes, and the sense 
of equilibrium. The floor of the metencephalon is part of the 
brain stem, and from it arises the pons, a bundle of axons con- 
necting the two sides of the cerebellum. The cavity is the 

The myelencephalon. The roof of the myelencephalon is 
covered by a thin roof plate, the chqroid plexus. Its floor forms 
the posterior portion of the brafn stem. ^The cranial nerves, 
from V to XII inclusive r <tepart from this portion of the stem, 
which merges imperceptibly into the spinal cord. Its cavity, 



hardly distinguishable from that of the metencephalon, is called 
the my^locogljSQ^th' ventricle). 

The spinal nerves. (^ ie nerves are segmentally arranged 
bundles of afferent and efferent neurons originally associated with 
the myotomes. The afferent neurons arise in the ganglia, the 
efferent in the floor of the spinal cord. Accordingly, a typical 
spinal nerve has two roots in the cord: a dorsal afferent root 
uniting with the ganglion; and a ventral efferent root which 
unites with the dorsal root after the other has attached itself to 
the ganglion YFig. 170). The nerve trunk then divides into 
branches, each containing afferent and efferent neurons, which 
are called rami and supply the body wall, although one (the com- 

Dorsal root 

Marginal layer 

Ependymal layer 
Mantle layer 

Somatic sensory neuron 
Visceral sensory neuron. 

Spinal ganglion 
Visceral motor neuron 

Somatic motor neuro: 
Dorsal ramus- 

Lat. terminal 

Ventral terminal division of 
spinal nerve 
Ramus communicans 

Sympathetic ganglion 

FIG. 170. Diagram to show the neuron components of a spinal nerve. Trans- 
verse section of 10 mm. human embryo. (From Arey after Prentiss.) 

municating ramus) connects with a sympathetic ganglion, derived 
from a spinal ganglion, through which the splanchnic afferent and 
efferent neurons serve the viscera. 

It has been shown by Coghill that the development of be- 
havior is closely paralleled by the development of the con- 
nections (synapses) between the neurons. Thus in the urodele, 
Ambystoma, the first reflex of the embryo, a bending away from a 
light touch on the skin, does not take place until an intermediate 



neuron in the spinal cord has established synaptic relations with 
the sensory tract on one hand and a floor plate cell which already 
has established a synaptic relation to the motor tract on the 
opposite side of the spinal cord (Fig. 171). 


FIG. 171. Diagram to show in transverse section of Ambystoma larva, neurons 
concerned in earliest reflex. (From Coghill, "Anatomy and the Problem of 

The autonomic nerves. The brain, spinal cord, and cranial 
and spinal nerves are grouped by anatomists as the central 
nervous system. Associated with this is the autonomic nervous 
system, consisting of nerves and ganglia and controlling the smooth 
muscles of the viscera and blood vessels, and some glands. This 
system arises from the neural plate, like the central nervous sys- 
tem, but from the lateral margins which become the neural crests. 
At the time when the neural crests are dividing into the cerebro- 
spinal ganglia, some of the cells migrate inward toward the dorsal 
aorta, where they aggregate and multiply to form the chain 
ganglia. The chain ganglia on each side become connected by 
fore and aft extensions which form the sympathetic trunks. They 
retain a connection with the cranial and spinal ganglia by means of 
the communicating rami, and send out nerves along the principal 
blood vessels. From the chain ganglia, by secondary and tertiary 



migrations, arise the prevertebral and visceral ganglia. In the 
head the four sympathetic ganglia (ciliary, sphenopalatine, otic, 



ganglion Vagus 


FIG. 172. Diagram to show migrations of autonomic ganglia in human develop- 
ment. (After Streeter.) 

and submaxillary) arise from the semilunar ganglion of the fifth 
cranial nerve, and later acquire connections with the chain 
ganglia (Fig. 172). 



It has already been noted (page 214) that some of the cells from 
the autonomic ganglia (chromaffin cells) migrate to the vicinity 
of the mesonephros to form the suprarenal gland. 

The cranial nerves. The cranial nerves, or nerves of the 
head regions, contain not only splanchnic and somatic afferent 

and efferent neurons compar- 
able to those of the spinal 
nerves, but also special affer- 
ent neurons from the nose, 
eye, ear and lateral line sys- 
tem. There are ten cranial 
nerves in the anamniotes, 
twelve in the amniotes (Figs. 
173, 174). To these should 
be added in all cases the ter- 
minal nerve, unknown when 
the cranial nerves were first 

O. Terminal, a ganglion- 
ated nerve from the organ of 
Jacobson entering the cere- 
bral lobe with functions un- 
known, probably sensory. 



I. Olfactory, a non-gangli- 
onated sensory nerve from 

FIG. 173. Diagram to show origin of cranial 
nerves in man. (After His.) 

the olfactory sensory region 
of the nose to the olfactory lobe. 

II. Optic (ophthalmic), a non-ganglionated sensory nerve from 
the retina of the eye to the floor of the diencephalon where the 
fibers from the two eyes cross (optic chiasma). Each set of 
fibers then enters the brain and runs to the optic lobe on the op- 
posite side of the brain to that on which the eye is located. 

III. Oculomotor (motor oculi), a motor nerve, somatic with 
some sensory elements, from the floor of the mesencephalon to all 
muscles of the eyeball except the superior oblique and the lateral 

IV. Trochlear, a motor nerve, somatic with some sensory ele- 
ments, from the roof of the mid-brain to the superior oblique 
muscle of the eyeball. 



V. Trigeminal, a mixed nerve. Its somatic sensory neurons 
arise in the semilunar ganglion, the motor elements in the floor of 
the myelencephalon. The sensory neurons are somatic (general 
cutaneous). The motor neurons supply the jaws (mandibular 

VI. Abducens (pathetic), a somatic motor nerve with some 
sensory elements, arising from the myelencephalon and supply- 
ing the external rectus muscle of the eyeball. 

VII. Facial, a mixed nerve. The afferent neurons arise in the 
geniculate ganglion and are splanchnic in nature, supplying the 


Somatic sensory 

Visceral sensory 

Somatic motor 

Visceral motor 

FIG. 174. Diagram showing relationships between cranial nerves and parts sup- 
plied. A, B, C, head somites. Arabic numerals, visceral arches. Roman 
numerals, nerves. 

hyoid arch, and also the tongue of mammals. In the anam- 
niotes, an associated ganglion gives rise to a lateral branch with 
afferent components from the lateral line organs. The efferent 
neurons supply the hyoid arch in the lower vertebrates and the 
facial region in the amniotes. The rami of the fifth and seventh 
nerves are closely associated. 

VIII. Acoustic (auditory), a ganglionated sensory nerve arising 
from the acoustic ganglion and bearing afferent neurons from the 
ear. In higher vertebrates it becomes differentiated into the 


vestibular and cochlear nerves, each with its own ganglion 
produced by the division of the acoustic ganglion. 

IX. Glossopharyngeal, a mixed nerve. The afferent neurons 
arise in the petrosal and the superior ganglion and are principally 
splanchnic. They divide into a prebranchial branch running into 
the hyoid arch and a postbranchial branch into the first branchial 
arch. The efferent components are principally found in the 
postbranchial branch. 

X. Vagus, a mixed nerve arising by the fusion of several primi- 
tive cranial nerves, which supplied the arches with afferent 
(from the jugular ganglion) and efferent neurons. In addition, 
the vagus gives off a visceral branch to the stomach, lungs, etc., 
and in the anamniotes a lateral branch to the lateral line organs 
of the trunk (from the nodosum ganglion). 

XI. Accessory, a motor nerve which innervates the muscles of 
the shoulder girdle and is found only in the amniotes. A ganglion 
(of Froriep) disappears before the embryo becomes adult. 

XII. Hypoglossal, also a motor nerve, which innervates the 
tongue in the amniotes. In the anamniotes the tongue is inner- 
vated by so-called " occipital " nerves which possibly are the 
fore-runners of the hypoglossal, prior to the appropriation of the 
occipital region by the head. 

Metamerism of the nervous system. The metameric ar- 
rangement of the nerves, like that of the segmental arteries, is 
purely secondary and dependent upon the primary metamerism 
of the mesoderm. The nerves, however, are more conservative 
than the vascular organs or myotomic derivatives. For ex- 
ample, the diaphragm of mammals is supplied by muscles from 
one of the cervical myotomes, and the innervation of the dia- 
phragm (phrenic nerve) still arises from the cervical region. 
Many attempts have been made to reconstruct the metamerism 
of the head, by a study of the cranial nerves, following Bell's law: 
that every original cranial nerve has, like a spinal nerve, a dorsal 
sensory and ventral motor root. 

This problem has been complicated by the fact that in the 
head there are two types of metamerism, (1) primary as indicated 
by the head myotomes in the elasmobranch embryo, and (2) 
secondary (branchiomeric) as indicated by the visceral arches 
(Fig. 174). Accordingly, there are two types of musculature, 














of eyeball 


of eyeball 



of jaw 


of eyeball 



Hyoid and 
facial movement 
and salivation 


Hearing and 


Taste and 




Movement of 
viscera and 


Movement of 
pharynx and 


of tongue 


(1) somatic as represented by the muscles of the eyeball, and (2) 
splanchnic as represented by the muscles of the jaws and visceral 
arches. Two types of efferent neurons, therefore, are present, 
(1) somatic and (2) splanchnic. The splanchnic motor neurons 
of the cranial nerves differ from those of the trunk, however, in 
that no sympathetic neurons intervene between them and the 
muscles which they supply. There are altogether three sets of 
afferent neurons: (1) the general sensory or cutaneous, which 
correspond to the somatic sensory neurons of the trunk; (2) 
splanchnic sensory, which correspond to those of the trunk; and 
(3) lateral, belonging to the lateral line system. The cranial 
nerves are evidently not serially homologous, as can be seen from 
Table 11. 

Finally, we must mention the neuromeres which have been 
reported in various vertebrate embryos. These are formed by 
constrictions in the cranial portion of the neural tube and inter- 
preted by some authors as the remains of a neural metamerism. 
They seem in many forms to correspond with the cranial nerves 
and more probably represent areas of local growth prior to the 
outgrowth of the nerves themselves. 

The general problem of the metamerism of the head still awaits 
solution. The latest summary, that of Brachet, indicates the 
probable number of segments in the primitive head as six. Three 
of these are ephemeral, and their somites give rise to mesenchyme. 
The three posterior segments are associated with the first three 
visceral clefts bounded by the first four arches, each of which has 
its own cranial nerve : the trigeminal of the mandibular arch ; the 
facial of the hyoid; the glossopharyngeal of the first branchial; 
the vagus of the second branchial arch. According to this inter- 
pretation, the posterior clefts and arches are reduplications sup- 
plied by new branches of the vagus, while the accessory and 
hypoglossal are secondarily acquired spinal nerves. 

THE FROG (SEE ALSO CHAPTER xi). The prechordal and epi- 
chordal divisions of the brain are demarcated by the notochord, 
and the division into the three primary vesicles is but slightly 
indicated. The brain of the frog never develops neuromeres. 
The optic lobes are corpora bigemina. The division into myelen- 
cephalon and metencephalon is incomplete, and no pons is formed. 
There are forty pairs of spinal nerves in the tadpole, reduced to 


ten in the adult. There are but ten of the cranial nerves (XI and 
XII not included). The sympathetic ganglia originate from the 
cranial and spinal ganglia by the emigration of ganglion cells. 

THE CHICK (SEE ALSO CHAPTER xii). The divisions of the 
brain into the three primary and five secondary vesicles is well 
marked. Eleven neuromeres are formed, of which three are found 
in the prosencephalon, two in the mesencephalon, the remainder in 
the rhombencephalon. ^Three flexures are formed: (1) cranial 
in the floor of the mesencephalon; (2) cervical at the junction 
of the myelencephalon and the spinal cord; and (3) pontine 
in the floor of the myelencephalon. A pons is formed. There 
are fifty pairs of nerves developed in the chick of eight days 
(Lillie), of which thirty-eight are spinal and twelve cranial, in- 
cluding the eleventh and twelfth which are not incorporated in 
the head of the frog. 

MAN (SEE ALSO CHAPTER xiii). The particular feature of im- 
portance in the development of the human brain is the great 
increase in size and complexity of the cerebral hemispheres of 
the telencephalon. The optic lobes are quadripartite (corpora 
quadrigemina), of which the two anterior lobes are especially 
associated with vision, the two posterior ones with hearing. 


The nervous system receives stimuli not only from outside the 
body but also from within, such as those concerning the tension of 
the muscles. For the reception of stimuli, special organs the 
sense organs are developed. Of these the most conspicuous 
are the eyes, the ears, and the nose. In addition, it must be 
remembered that the entire skin functions as a sense organ by 
means of special receptors, and that stimuli are received from the 
viscera and other internal structures by means of free nerve 

Of the special sense organs, the eye is most specialized in its 
mode of development. It is responsive to photic stimuli. v The 
nose represents a concentration of chemical sense receptors, more 
highly developed than the scattered taste buds of the head, 
which are confined in adult mammals to the cavity of the mouth. 
The ear, responsive to slower vibrations (pressure, sound) in the 
surrounding medium, originates in a manner similar to that of 





the lateral line system. This system is highly developed in 
the aquatic anamniotes, vestigial or absent in the amniotes. The 
ear, on the other hand, is more highly developed in the amniotes. 
The nose, -f- The nose arises as a pair of local thickenings of the 
ectoderm at the anterior end of the head (Fig. 175). These 
thickenings are hereafter known as the nasal (olfactory) placodes. 
Later they invaginate to form the nasal (olfactory) pits,)which 
persist as the nose of all fish except the air-breathing dipnoi. 
Here also should be noted the fact that the cyclostomes are 
peculiar in the possession of a single median nasal pit. Among 
the tetrapods^hc nasal pits elongate to become oro-nasal grooves, 
the anterior ends of which become connected with the developing 
mouth into which they are carried. N The original anterior ends 






FIG. 176. Sagittal hemi-section through human nose. (After Howden.) 

of the nasal pits, therefore, come to lie at the posterior end of the 
mouth and open into the pharynx as the internal nares, while the 
original posterior ends become the external nares (Fig. 175E). 
The nasal cavity is later separated from the oral cavity by the 
ingrowth of the maxillary, palatine, and pterygoid bones, which 
form the hard palate ^Fig. 176). Jacobson's organ arises as a 
pocket of the olfactory epithelium. Its function is unknown. 
The olfactory epithelium contains ciliated cells connected to the 
olfactory lobe by means of the first cranial nerve) which is 


peculiar in that its neurons run directly to the brain without the 
interposition of ganglion cells. Jacobson's organ receives a 
branch of the trigeminal nerve. 

The eye. The optic placodes are incorporated into the neural 
plate, where they can be distinguished as lateral thickenings of 
the margin at points which will later be included in the dienceph- 
alon (Fig. 177). When the tube is formed, the relation of the 
sensory epithelial cells to the exterior is, of course, reversed. 
The optic placodes " invaginate," but, owing to their relation 
to the neural tube, the result is an apparent " evagination " from 
the tube towards the exterior. This produces the outgrowths 
which later, by constriction, give rise to the proximal optic stalks 
and distal optic vesicles. At the point where the optic vesicle 
touches the ectoderm, two reactions take place: (1) a local thick- 
ening of the ectoderm, called the lens placode, from which the lens 
of the eye develops; and (2) an invagination of the optic vesicle 
whereby this vesicle is transformed into a two-layered optic cup 
This invagination continues into the optic stalk to produce a 
groove called the choroid fissure. 

The lens. -4 The lens placode invaginates to form the lens pit, 
which then withdraws still further from the surface and becomes 
closed in by the union of its external lip to form the lens vesicle. 
The lensVesicle becomes solid by the elongation of the cells on the 
internal side which assume a clear transparent appearance .J) 

The optic cup. The inner layer of the cup becomes the sen- 
sory portion of the retina, the outer layer the pigmented portion, 
it will be recalled that the sensory epithelium of the eye is in- 
verted, and as a result the rods and cones, or sensory elements, 
of the retina are pointed away from the light. 1 fin the pigmented 
layer of the retina, melanin is secreted. Meantime the cavity of 
the optic cup becomes filled with a clear fluid secreted by the 
surrounding cells, which later becomes viscous and forms the 
vitreous humorA 

The envelopes of the eyeball (Fig. 178). Around the optic 
cup and stalk, a layer of mesenchyme accumulates, which later 
differentiates into an inner delicate layer called the choroid which 
contains pigment and capillaries and may be compared with the 
pia mater of the brain, and an outer dense layer known as the 
sclera, which may be compared with the dura mater of the brain. 



The external portion of the sclera over the lens makes contact 

becomes transparent to form the cornea. 

FIG. 177. Diagrams showing early stages in development of vertebrate eye. A, 
optic placodes (in black). B, same after formation of neural tube. C, optic 
vesicles and lens placodes. D, optic cups and lens pits. PJ, optic cups and lens 

The epidermis over the eye forms the conjunctiva.^- In some 
vertebrates, sclerotic cartilage, or even bone, is formed, the ves- 
tige of an optic capsule. ) The edge of the choroid, together with 



the marginal retina, gives rise to the iris, a circular curtain sur- 
rounding the opening of the cup which is called the pupil of the 
eye. The muscles of the iris are apparently of ectodermal origin. 
The iris divides the space between the lens and the cornea into 
two chambers, an anterior and a posterior chamber, which are 
filled with a fluid, the aqueous humor. The muscles of the 


Anterior chamber 


Posterior chamber 




Optic nerve 

FIG. 178. Horizontal section of human eye. (After Howden.) 

eyeball are six in number, arising from the three head myotomes. 
They are innervated by the oculomotor, trochlear, and abducens 

The optic nerve. The afferent neurons pass from the retina 
into the optic cup and form a bundle which passes out through 
the choroid fissure and into the optic stalk, and so to the optic 
chiasma on the floor of the diencephalon, where they cross and 
make their way to the optic lobes on the opposite side. 

The lateral line system. This is a diffuse sensory organ con- 
sisting of sense buds arranged in rows over the head and body of 
aquatic anamniotes. Its function apparently is to detect slow 
vibrations in the water. The origin of the lateral line system is a 
lateral thickening of the sensory ectoderm which later breaks up 
into separate suprabranchial placodes. These are found in the 


embryos of the amniotes but soon degenerate. The lateral line 
system is of particular interest inasmuch as the lateral thicken- 
ing referred to is in some cases continuous with the otic placode 
which gives rise to the ear. The principal nerve supplying the 
lateral system is the facial, although trigeminal, glossopharyn- 
geal, and vagus often contain lateral line components. 

The ear. The ear becomes differentiated into the vestibule or 
equilibratory organ and the cochlea or organ of hearing. Three 
parts of the ear are distinguished (Fig. 180). The inner ear, 
giving rise to the vestibule and the cochlea, arises from an ecto- 
dermal otic (auditory) placode. The middle ear appears in the 
amphibians, and it is derived from the endodermal first visceral 
pouch. The outer ear, found only in the amniotes, is an ecto- 
dermal derivative of the first visceral groove and an outgrowth 
from the mandibular and hyoid archest >^ 

The inner ear. This originates from the otic placode, which 
invaginates to form an otic (auditory) pit (Fig. 179) and later 
closes over to withdraw from the epidermis as the otic (auditory) 
vesicle or otocyst. , In some vertebrates (elasmobranchs) the 
vesicle retains its connection with the exterior by means of a 
hollow stalk, the endolymphatic duct. Usually this connection 
is lost and the endolymph duct of the adult is a new formation. 
The vesicle divides into a ventral saccule and a dorsal vestibule 
or utricle. The saccule gives rise to the^ndolymph duct and the 
lagena, which in mammals becomes th coiled cochlea or organ of 
hearing, while the utricle gives rise by constriction to three semi- 
circular canals, each with a dilation at one end^the ampulla. 
The sensory epithelium is restricted to the lagena and ampullae^ 
The cavity of these structures is known as the membranous 
labyrinth, and contains a fluid, the endolymph. Concretions, the 
otoliths, may appear in the endolymph of the vestibular portion. 
Around this labyrinth vthe otic capsule, )or skeletal labyrinth^ is 
formed. This later ossifies to give rise to the otic bones. iThe 
skeletal labyrinth contains a fluid known as the perilymph. In 
vertebrates with a middle ear^two openings are formed in the 
skeletal labyrinth, the fenestra rotunda, closed by a membrane, 
and the feQestra o vale, ~ Into which the stapes projects.! The 
acoustic nerve, which is ganglionated, divides into a vestibular 
and a cochlear nerve, each with its separate ganglion. } 



FIG. 179. Diagrams showing early stages in development of inner ear, A, otic 
placodes (in black). B, otic pits. C, otic vesicles (otocysts). 





FIG. 180. Frontal section of human ear. Semi-diagrammatic. (After How den.) 


The middle ear. The middle ear arises from the first visceral 
Rouch, which constricts into a proximal auditory (Eustachian) 
tube and a distal tympanic cavity which is separated from the 
exterior by the tympanic membrane,(a persistent closing plate 
formed from ectoderm and endoderm. Through the tympanic 
cavity there is a chain of bones (auditory ossicles) connecting the 
tympanum with the fenestra ovalis. In anurans and sauropsids, 
this chain of auditory ossicles consists of the columella and stapes 
(hyomandibular). In the mammals, the columella is replaced by 
the incus and malleus, equivalent to two other jaw bones, the 
quadrate and articulare, respectively. The muscles of the middle 
ear, tensor tympani and stapedial muscles, arise from the meso- 
derm of the mandibular and hyoid arches, respectively, and are 
innervated by the facial and glossopharyngeal nerves. 

The outer ear. The external ear consists of the external 
auditory meatus, derived from the first visceral groove, and the 
pinna, which arises from tubercles on the mandibular and hyoid 
arches. It is composed of mesoderm and ectoderm, contains 
muscles, and is strengthened by cartilage. The innervation is 
from the facial nerve. 

THE FROG (SEE ALSO CHAPTER xi). In the development of 
the nose, the nasal groove stage is suppressed. Instead, a thick- 
ening develops from the olfactory pit into the mouth as far as the 
pharynx. This acquires a lumen which connects the olfactory 
pit to the pharynx. The development of the eye presents no 
especial peculiarities. The endolyrnph duct is a dorsal evagina- 
tion from the otocyst. The semicircular canals are each formed 
by the appearance of a pair of ridges in the cavity of the utricle 
which fuse to enclose the cavity of the canal. The saccule gives 
rise to two ventral diverticula, the cochlea and basilar chamber. 
The function of the latter is unknown. The tubo-tympanic cavity 
arises from the first visceral pouch, which in the frog is vestigial 
and has no cavity. From this rudiment a strand of cells grows 
dorsad and later acquires a lumen. It loses its connection with 
the pharynx and moves backward to the ear region where it 
acquires a secondary connection with the pharynx (Fig. 181). 
The tympanic membrane is apparently entirely ectodermal. The 
columella, which connects the tympanum with the inner ear, arises 
from two primordia: the inner stapedial plate, which is a part 


of the otic capsule; and a cartilage derived from the palato- 
quadrate bar. This cartilage is thought to be homologous with 
the hyomandibular bone of fishes. The lateral line organs arise 
from the fragmentation of a placode known as the placode of the 
tenth cranial nerve, which innervates this series. Similar epi- 
branchial placodes appear on the head arid are innervated by the 

Brain A j- Utriculus and 

Auditory sem i. c j rai i ar canals 
nerve / 




plate ' 

ipsule and 



FIG. 181. Rana pipiens, diagram to show the parts of the ear. Schematic cross- 
section through head. 

seventh and ninth nerves. They are larval sense organs and 
disappear at metamorphosis. 

THE CHICK (SEE ALSO CHAPTER xii). The chick has a cleft 
palate due to the incomplete fusion of the palatine processes of 
the maxillae. Jacobson's organ makes a short appearance as a 
vestigial organ but disappears before hatching. The eye pos- 
sesses three eyelids, the third (nictitating membrane) arising 
from a separate fold inside that which forms the upper and 
lower lids. The pecten is a vascular plate in the vitreous 
humor, from mesenchy^ne which enters the choroid fissure. Its 
function is unknown. CThe endolymphatic duct arises from the 
dorsal wall of the otocyst. The semicircular canals arise as out- 
pocketings of the otocyst prior to its separation into utricle and 
saccule. The cochlea is more highly developed than in th~e frog. 
The tubo-tympanic cavity arises from the first pharyngeal pouch. 
The tympanum is formed from ectoderm and endoderm and in- 
cludes a middle layer of mesenchyme. The columella arises from 


a stapedial plate and hyomandibular cartilage. The external 
auditory meatus is short, and no pinna is developed. ) 

MAN (SEE ALSO CHAPTER xiii). The organ of Jacobson is 
rudimentary and may completely disappear in the adult. A 
small fold (plica semilunaris) is the representative of the nictitat- 
ing membrane. The cochlea is highly differentiated. The tube 
and tympanic cavity form from the first visceral pouch. The 
tympanum apparently is composed of all three germ layers. 
There are three auditory ossicles. The stapes is derived from the 
second visceral arch, while the malleus and incus arise from the 
first visceral arch. They are thought to represent the quadrate 
and articular bones of reptiles, respectively. The pinna arises 
from elevations on the mandibular and hyoid arches. 


The ectoderm gives rise to the epithelial linings of the following 

A. The epidermis, with the 

apertures of 

Oral cavity 
Visceral clefts 

B. The neural plate 

1. Neural tube 

Brain and cranial nerves 
Cord and spinal nerves 

2. Neural crest 


Suprarenal gland 


C. Sensory placodes 

1. Nose 

2. Eye (choroid and sclera from mesoderm) 

3. Ear (middle ear from endoderm, ossicles from meso- 


4. Lateral line organs 


Arey, L. B. 1934. Developmental Anatomy, 3rd Ed., Chaps. 14-17. 
Brachet, A. 1921. Trait6 d'cmbryologie des vert6bres, Part II, Bk. 1, Chap. 4. 
/Coghill, G. E. 1929. Anatomy and the Problem of Behavior. 
Hertwig, O. 1906. Handbuch, Book II, Chaps. 5-10. 
Jenkinson, J. W. 1913. Vertebrate Embryology, Chap. 7. 
Keibel and Mall. 1910-1912. Human Embryology, Chaps. 14 and 16. 
Kerr, J. G. 1919. Textbook of Embryology, Chap. 2. 
Kingsley, J. S. 1926. Comparative Anatomy of Vertebrates. 
Lillie, F. R. 1919. The Development of the Chick, 2nd Ed. 
MoMurrich, J. P. 1923. The Development of the Human Body. 
Strong, O. S. 1921. The Nervous System, being Chap. 17 of Bailey and Miller, 
Textbook of Embryology, 4th Ed. 



In earlier chapters we have discussed the fertilization of the 
frog's egg (page 57), its cleavage (pages 97, 103), and germ-layer 
formation (pages 109, 118), and have observed that while the germ 
layers are being laid down the process is complicated by the early 
localization of some of the organ systems, notably the sensory- 
nervous complex (page 129). In this account of later organog- 
eny, three stages of development seem especially significant: 
first, an early embryo of about 3 mm. body length in which the 
visceral grooves are apparent, a stage attained in Rana pipiens 
about the second day after the eggs are laid; second, the newly 
hatched larva of about 6 mm. with external gills developing, 
about two weeks old; third, a young " tadpole " stage of about 
11 mm. with the opercula covering the internal gills, about the 
age of one month. 

These stages are easily identified even though the lengths and 
ages can be given only approximately, for the rate of develop- 
ment is greatly influenced by the prevailing temperature, and 
the size of the tadpole is determined largely by external factors, 
such as the amount of food available. 

The student must bear in mind that the sections illustrated in 
this and the two chapters following are for the sole purpose of giving 
him starting points from which he is expected to study all the sections 
in the series furnished him. He will probably never encounter 
sections exactly like those selected for these illustrations, but he mil 
discover sections very like them from which he can commence his 
own observations. 


External form. This stage corresponds approximately to the 
embryo of 3^ mm. described by Marshall. The head region, 
through its more rapid growth, has become easily distinguishable 
from the trunk, which bulges ventrally on account of the large 
amount of contained yolk, and a well-marked tail bud is present. 




The neural folds have fused throughout their length, and en- 
closed the blastopore. In the head the stomodeum appears 
as an antero-posterior slit on the anterior ventral surface, and is 
enclosed by ridges identifiable as the maxillary processes and 
mandibular arches. On either side and slightly ventral to the 
stomodeum, are the primordia of the sucker or oral gland. At the 
dorso-lateral margins the olfactory placodes have begun to evagi- 
nate. Lateral bulges on either side of the head are due to the 
developing optic vesicles. The ear is now in the otic vesicle stage. 
The gill region shows five visceral grooves. Immediately behind 
the last arch, a swelling is caused by the developing pronephros. 
Dorsally, slight furrows indicate the boundaries of thirteen so- 
mites. Beneath the tail 

Mesencephalon I Optic vesicle 

Otic vesicle 

Somite IH- 

Neural tube 



-Visceral pouch I 
Fore gut 

Mid gut 

Hind gut 

bud, the proctodeum 
has united with the 
hind-gut to form the 
cloacal aperture. 

Endodermal deriva- 
tives. The anterior 
portion of the gastrocoel 
is now a large fore-gut 
with a thin-walled 
lining. From this, on 
either side, the begin- 

. ' e 

nings of three visceral 

h ^ can be ^^ 

r rom the fore-gut a nar- 
row evagination grows backward into the floor of the mid-gut as 
the primordium of the liver. The mid-gut is distinguishable by 
its relatively narrow lumen and thick yolk-laden floor. The 
small but thin-walled hind-gut opens above into the neurenteric 
canal by which it is connected with the neurocoel, and opens 
ventrally to the exterior by way of the proctodeum. An axial 
rod, the hypochord, is found beneath the notochord. It origi- 
nates from the roof of the gastrocoel and disappears soon after 

Mesodermal derivatives. The notochord is large and vacuo- 
lated and enclosed by two sheaths. The somites have now 
attained their maximum number (13) in the trunk, but are not 

r XT - . . 



FIG. 182 -3 mm. frog embryo, viewed from right 
side as a transparent object. X15. 



yet distinguishable in the tail region. The intermediate meso- 
derm, after a temporary division into nephrotomes, is now re- 
united into a nephrotomal band in which spaces have appeared 
opposite the second, third, and fourth somites, indicative of the 
pronephric tubules which are to develop. A thickening along the 



Fore gut 







FIG. 183. 3 mm. frog embryo. 

Hind gut 
Sagittal section. 



nephrotomal band immediately below the ventro-lateral margins 
of the somites is the primordium of the pronephric duct. Imme- 
diately below the floor of the fore-gut, the lateral mesoderm has 
separated into dorsal splanchnic and ventral somatic layers, while 
the contained space is the beginning of the pericardial cavity, the 
only region of the coelom yet apparent. 

Ectodermal derivatives. The epidermis at this stage is 
ciliated. The neurocoel, as has been remarked above, is con- 



nected with the hind-gut by the neurenteric canal. At the 
anterior end, the brain is distinguishable by its relatively larger 
lumen and by the cranial flexure over the anterior end of the 
notochord. The divisions between the three primary vesicles 

are not marked by the con- 
strictions characteristic of 
many vertebrates, but are 
distinguished by the follow- 
ing points of reference : the 
prosencephalon extends to a 
line projected from a thick- 
ening on the floor known as 
the tuberculum posterius to 
a point just in front of a 
similar thickening on the 
dorsal wall; the mesenceph- 
alon, from the boundary of 



Oral gland 

FIG. 184. 3 mm. frog embryo. Transverse 
section through optic vesicle. X50. 

the prosencephalon to a line 
connecting the tuberculum 
and a point just behind the 
dorsal thickening; the rhombencephalon merges imperceptibly 
into the spinal cord. From the prosencephalon, the optic vesicles 
extend on either side and cause the external bulges already noted. 
From the ventral side of the prosencephalon, a depression, the 
infundibulum, extends towards the hypophysis, which in the frog 
grows inward as a solid wedge of ectodermal cells anterior to 
the stomodeum. Dorsally, the epiphysis appears as a median 


External form. Although the larva, if it may be so called, 
has emerged from the protecting membranes of egg jelly, the 
mouth has not yet opened and for several days the yolk is still 
the sole source of food. The head region is still easily distin- 
guishable from the trunk, while the tail has increased greatly in 
length and has become bilaterally compressed. In the head, the 
stomodeal pit has deepened at the anterior end, and the maxil- 
lary processes and mandibular arches are more sharply sculp- 
tured. The invagination of the nasal (olfactory) placodes has 








Oral gland 

FIG. 185. 3 mm. frog embryo. Transverse section through otic (auditory) 

vesicle. X50. 


Neural tube 




FIG. 186. 3 mm. frog embryo. Transverse section through mid-gut and liver. 




Prosencephalon $ 

he.' '// 


jfir Hind-gut 

FIG. 187. 3 mm. frog embryo. Frontal section through optic stalks, liver, and 

hind-gut. X50. 



continued to the point where they may be called pits, connected 
to the anterior margins of the stomodeal pit by oro-nasal grooves. 
The bulge of the eye is still prominent. The primordia of the oral 
glands have fused to form a U-shaped sucker ventral and pos- 
terior to the stomodeum. The visceral grooves are still sepa- 
rated from the visceral pouches by closing membranes, while on 
the third and fourth arches external gills have appeared. Behind 
them the pronephric elevation is well marked, and continues 
backward as a slight ridge marking the pronephric duct. 
Intersomitic grooves are still apparent. On the ventral side 
at the base of the tail is the cloacal aperture. 



Otic vesicle 




Oral gland 


External gills 



FIQ. 188. 6 mm. frog larva (just hatched). Transparent preparation, viewed from 

right side. X15. 

Endodermal derivatives. On either side of the fore-gut are 
to be seen five visceral pouches, although they would hardly be 
recognized as such since they are so compressed. A groove on 
the ventral side of the pharyngeal cavity is the primordium of 
the thyroid gland. At this stage, also, the dorsal epithelial 


bodies of the first two visceral pouches (hyomandibular and first 
branchial) may be distinguished. The liver diverticulum has 
increased in length. The hind-gut has lost its connection with 
the neurocoel through the occlusion of the neurenteric canal, but 
now receives the posterior ends of the pronephric ducts. 

Mesodermal derivatives. The notochord has grown back 
into the tail. The somites have now become differentiated into 
the myotomes, dermatomes, and sclerotomes, while from the 
myotomes muscle cells have been formed. The pronephros is 
now established. There are three pronephric tubules, each 
opening into the coelom by means of a ciliated nephrostome. 
Opposite these, a mass of capillaries, connected with the dorsal 
aorta, forms the so-called glomus, equivalent to the separate 
glomeruli of other vertebrates. The pronephric tubules grow 
backward into the pronephric ducts, which have acquired lumina. 
At the time of hatching, the primordia of the heart have fused 
to form a tube, twisted slightly and almost S-shaped, suspended 
in the pericardial cavity by a dorsal mesocardium. Two regions 
may be distinguished, the posterior atrium and anterior ven- 
tricle. From the ventricle leads the bulbus, arising from the 
fusion of paired primordia. This connects with the dorsal aorta, 
also the result of fusion, by means of aortic arches in the third 
and fourth visceral arches (vestiges of the first and second aortic 
arches have already appeared and disappeared). At a slightly 
later stage, loops from these arches will grow out into the ex- 
ternal gills to form a branchial circulation. The anterior ends of 
the dorsal aortae are prolonged to form the internal carotids, 
while the posterior ends unite directly above the heart, and just 
after uniting give off the glomi on either side. Both the somatic 
and splanchnic venous systems are represented at this stage. 
Two vitelline veins unite to enter the heart at the sinus venosus. 
The cardinal veins at this time are represented by irregular 
lacunar spaces in the head and near the pronephros. 

Ectodermal derivatives. The epidermis is still ciliated. 
From the prosencephalon the thin-walled cerebral vesicle has 
appeared. The epiphysis is well marked, and the infundibulum 
is in contact with the hypophysis. At this time the primordia 
of cerebrospinal nerves may be distinguished. In the spinal 
nerves, dorsal roots arise from the ganglia produced by the seg- 








Spinal canal 


Hind gut 

FIG. 189. 6 mm. frog larva. Sagittal section, anterior portion. X50. 







Fore-gut * 

Oral gland 



FIG. 190. 6 mm. frog larva. Transverse section through optic cup. X50. 





Otic vesicle 

arch HI 


-.- Ventricle 

Oral gland" 

FIG. 191. 6 mm. frog larva. Transverse section through otic vesicle. X50. 



mentation of the neural crest while the ventral roots arise from 
neuroblasts in the spinal cord. In the head, four ganglia arise 
and with each is associated a placode of nervous ectoderm. From 
the first ganglion and placode, the trigeminal (V) nerve arises. 
The second combination gives rise to the facial (VII) and acoustic 
(VIII) cranial nerves, while the remainder of this placode in- 
vaginates to form the otic vesicle. The third ganglion and pla- 
code produce the glossopharyngeal (IX) cranial nerve, and the 


Spinal cord 




FIG. 192. 6 mm. frog larva. Transverse section through pronephros. X50. 

fourth gives rise to the vagus (X). The fourth placode grows 
back as far as the tail, giving off as it goes small groups of cells 
which later become the lateral line organs of the trunk. Those 
of the head arise from the second and third placodes. At this 
time, also, ganglion cells are migrating toward the dorsal aorta to 
aggregate as the ganglia of the autonomic nervous system. The 
eye is well advanced in development, as the optic vesicles have 
invaginated to form the optic cup and the lens placode has sepa- 
rated from the epidermis and acquired a cavity. The ear is in 
the otic vesicle stage with an endolymphatic duct. The nose is 
still represented by the nasal pits. From the prolongation of 
the fourth placode referred to above, the lateral line system is 
in process of formation. 



I Visceral 


Mid -gut 


FIG. 193. -6 mm. frog larva. Frontal section through nasal pit and visceral 

pouches. X50. 



External form. The head and trunk are now fused into a 
common ovoid mass, sharply distinguished from the long bi- 
laterally compressed tail. The mouth is open and equipped with 
horny raspers, while the oral gland is reduced to two vestiges 
on the ventral side of the head. On the dorsal surface, the large 
eyes, now functional, protrude slightly. Anterior to these are 
the external openings of the nasal tubes (external nares). The 
external gills, which were developing at hatching, have now de- 
generated and been replaced by internal gills concealed from view 
by the opercula. On the left side, the opercular aperture serves 
as a means of egress for the water from which the gills obtain 
their oxygen. The tail, now two-thirds the length of the tad- 
pole, has a dorsal and a ventral fin. Close to the margin of the 
latter, at the base of the tail, is the cloacal opening. 

Endodermal derivatives. The mouth has been formed by 
the breaking through of the oral membrane. From the pharynx, 
all the visceral pouches except the hyomandibular and the 
vestigial sixth pouch open to the exterior as visceral clefts; and 
demibranchs have arisen on the anterior and posterior margins of 
the third, fourth, and fifth visceral arches and on the anterior 
margin of the sixth. These are the internal gills which hang down 
into the opercular cavity. The epithelial bodies from the hyoman- 
dibular pouch have disappeared. Those from the second pouch 
form the thymus gland, while similar buds arise from the third 
and fourth but presently disappear. The ventral epithelial 
bodies of the second pouch are said to give rise to the carotid 
gland, and those of the third and fourth to " parathyroids." 
The fifth pouch never gains communication with the exterior 
but gives rise to the ultimobranchial bodies. The thyroid is 
now separated from the pharynx. In the tadpole the pulmonary 
organs consist of a pair of thin-walled sacs, the lungs, arising 
from a laryngeal cavity connected with the pharynx by a narrow 
opening, the glottis. Posterior to the pharynx comes the esopha- 
gus, which was occluded just before the opening of the mouth but 
now possesses a narrow lumen opening into the stomach, which is 
not greatly dilated. The vesicle, which formerly represented the 
liver, persists as the gall bladder and common bile duct, rela- 








FIG. 194. 11 mm. frog larva. 1 Transparent preparation viewed from right side. 


1 Figs. 194-198 inclusive are from preparations loaned me by Dr. A. R. Cahn. 
In earlier editions they were labelled 9 mm., as measured after preservation. 







Dorsal aorta- 

of tail 




FIG. 195. 11 mm. frog larva. Sagittal section, anterior part. X40. 



tively small in comparison with the great glandular mass of the 
liver. Although the pancreas arose from paired primordia of the 
duodenum, these have now shifted their position so that their 
ducts open into the common bile duct. The intestine is ex- 
tremely long and coiled into a double spiral. It terminates in a 
slightly dilated rectum, opening into the cloacal cavity which also 
receives the pronephric ducts and opens to the exterior by the 
cloacal aperture. 

Mesodermal derivatives. The notochord has elongated to- 
ward the posterior end, accompanying the growth of the tail. 
The two most anterior somites have disappeared, leaving eleven 
in the trunk region and a much larger and variable number in 



ry layer 

Pigment layer 

FIG. 196. 11 mm. frog larva. Transverse section, through eye. X40. 

the tail. In the tail the myotomes have given rise to the dorsal 
and ventral musculature. The pronephros has become larger 
and more complicated through the branching of the pronephric 
tubules. The coelom consists of a pericardial cavity containing 
the heart, whose myocardia have disappeared, and an abdominal 
cavity in which the gut is suspended by the dorsal mesentery. 
These cavities are still continuous up to the time of metamorpho- 
sis. In the heart the sinus venosus is now a large transverse sac; 
the atrium is partially divided by the interatrial septum; the 
ventricle has thick muscular walls; and the short bulbus opens 
into the ventral aorta which is divided into proximal and distal 
portions by a pair of valves. The ventral aorta is divided into 



four afferent branchial arteries, the ventral portions of aortic 
arches III-VI. From these the blood passes through the internal 
gills by means of capillaries and is conveyed to four efferent 
branchial arteries, the dorsal portions of the aortic arches referred 
to above, which in turn lead to the dorsal aortae. The carotid 
arteries are connected in front of and behind the infundibulum by 
commissural vessels, and continue forward as the anterior cere- 
bral arteries. From the anterior commissure the basilars run 
backward and the anterior palatines forward. The pharyngeal 

Otic vesicle 








* Opercular 

FIG. 197. 11 mm. frog larva. Transverse section through ear. X40. 

artery, running forward from the dorsal aorta, at a point just 
posterior to the anterior commissure, represents the dorsal portion 
of the mandibular arch; the lingual artery arises independently 
and unites with the first efferent branchial. From the efferent 
branchial arteries of the sixth arch, the pulmonary arteries grow 
backward to the lungs. The vitelline veins have been broken 
up, by their inclusion in the developing liver, into hepatic veins, 
opening into the sinus venosus, and hepatic-portal veins from the 
intestine. The anterior cardinal veins are formed by the union 
of the superior jugular and facial veins and empty into the com- 
mon cardinals. From the ventral side of the head the inferior 
jugulars drain into the common cardinals. The posterior somatic 
veins are the posterior cardinals, which return the blood from the 



region of the pronephros into the common cardinals. The lym- 
phatic vessels of the tadpole have arisen from the confluence of 
numerous, small intercellular spaces in the mesenchyme. 

Ectodermal derivatives. The epidermis is no longer ciliated. 
The cerebral vesicle is now subdivided into right and left por- 
tions, while immediately behind this is the choroid plexus of 
the diencephalon. The pineal gland is connected with the 
diencephalon by a small stalk; the pituitary gland has lost all 
connection with the exterior. In the mesencephalon the optic 

Neural tube 


Gall bladder 





FIG. 198. 11 mm. frog larva. Transverse section through pronephros. X40. 

lobes are just apparent. The metencephalon is distinguishable 
by the thickness of its walls as compared with the choroid plexus 
of the myelencephalon. All cranial nerves and spinal nerves 
are now established. The eye now contains all elements neces- 
sary for functioning; rods and cones of the sensory layer connect 
with the neurons of the optic nerve; pigment is deposited in 
the pigment layer; the choroid and sclerotic layers have been 
formed from mesenchyme; the lens is transparent, as is the 
cornea formed from the ectoderm. The otocyst is partially 
divided by a dorsal partition into an outer saccule and inner 
utricle. The nasal pits have grown backward as solid rods 
which by now have acquired lumina and will soon open into the 




Nasal pit 



Wall of 

.Muscle mass 

Spinal cord 

FIG. 199. 11 mm. frog larva. Frontal section through nose, eye, and ear. X40. 


Huxley, J. S., and de Beer, G. R. 1934. The Elements of Experimental Embry- 
ology, Chap. 2. 

Jenkinson, J. W. 1913. Vertebrate Embryology, Chap. 7. 

Kellicott, W. E. 1913. Chordate Development. 

Marshall, A. M. 1893. Vertebrate Embryology, Chap. 3. 

McEwen, R. S. 1931. Vertebrate Embryology, 2nd Ed., Part 2. 

Morgan, T. H. 1897. The Development of the Frog's Egg. 

Zeigler, H. E. 1902. Lehrbuch der vergleichenden Entwickelungsgeschichte der 
niederen Wirbeltiere. 



The traditional stages in the development of the chick (Gallus 
domesticus) for laboratory practice are those at the end of each 
of the first three days of incubation. So many important changes 
take place during the second day, however, that it is advisable 
to study an additional stage intermediate between twenty-four 
and forty-eight hours in age. The chick of thirty-three hours 
is selected because the form of the embryo is not yet affected by 
torsion or flexure, and the headfold of the amnion has not yet 
slipped over the head of the chick. 

As it is a well-known fact that, in these first few days of incuba- 
tion, embryos of the same age have attained varying degrees of 
development, the length of the embryo has been proposed as a 
mark of identification. The flexures of the body, however, 
make this standard impracticable, and the remaining alternative 
is to select the specific development of some particular structure 
as a basis of arrangement. For this purpose the number of 
somites, suggested by Lillie, is admirable. Still, it must be re- 
membered that on account of the effect of temperature upon the 
rate of development, the number of somites is not correlated 
exactly with the number of hours of incubation, as may be seen 
fronj the following table. 






About 24 hours 

Fig. 76 


Fig. 9, 9A 

(24 hrs. 7-8 S) 

Fig. 59 

(25 hrs. 78) 

Fig. 36 
(27 hrs. 8 S) 

About 33 hours 

Fig. 93 
(33 hrs. 16 S) 

Fig. 10, 10A 
(32 hrs. 9 S) 

Fig. 63 
(33 hrs. 12 S) 

Fig. 39 
(33 hrs. 12 S) 

About 48 hours 

Fig. 109 

(48 hrs. 27-28 S) 

Fig. 16, 16A 
(52 hrs. 27 S) 

Fig. 93 
(48 hrs. 27 S) 

Fig. 55 

(55 hrs. 29 S) 

About 72 hours 

Fig. 115 

(68 hrs. 37 S) 

Fig. 18, 18A 
(67 hrs. 35-37 S) 

Fig. 117 
(72 hrs. 35 S) 

Fig. 63 
(72 hrs. 36 S) 





At the end of the first day of incubation, the chick embryo has 
completed the period of cleavage (pages 98, 105) and germ-layer 
formation (pages 111, 121), and is in the early stages of orga- 

Anterior neuropore 

Head fold 




intestinal portal 
"" Neural fold 






FIG. 200. 24 hour chick embryo. Cleared preparation from dorsal side. X25. 

External form. The embryo, 3.3 mm. in length, lies along 
the axial line of the slipper-shaped area pellucida which in turn is 
surrounded by the crescent-shaped area vasculosa, whose anterior 
horns, separated by the proamnion, reach about to the level of 



Ectoderm \I 



tip of the head. At the anterior end, the head fold of the embryo 
is lifted above the proamnion from which it is separated by the 
subcephalic pocket. In the head fold is contained the fore-gut, 

0.59 mm. in length, which opens at its 
posterior end into the yolk cavity by 
means of the an terior_ intestinal portal. 
On either margin of the portal the pri- 
mordia of the vitelline veins are to be 
recognized in thick bands of splanchnic 
mesoderm. The neural plate has al- 
ready given rise to the neural folds 
which extend back as far as the first 
somite. They have united just posterior 
to the region where the optic vesicles are 
to appear and thus have given rise to a 
neural tube 0.3 mm. in length, which is 
widely open in front and behind as the 
anterior and posterior neuropores, re- 
spectively. Behind the head fold the 
axial mesoderm is segmented into six 
somites. Between the neural folds the 
notochord can be recognized as a faint 
line which joins, at its posterior end, the 
primitive streak, now reduced to 0.83 
mm. in length. 

Endodermal derivatives. The only 
differentiation which has taken place in 
the endoderm consists of the establish- 
ment of the fore-gut by means of the 
folding off of the head from the proam- 
nion. As this process continues the fore- 
gut will be lengthened at the expense of 
FIG. 201. 24 hour chick em- the widely open mid-gut, and the an- 

X37i. Saglttal SeCti n ' terior in testinal portal will progress 

steadily backward. 

Mesodermal derivatives. The mesoderm proper does not 
extend into the head, but a loose aggregate of mesenchyme 
derived from it is present. Posterior to the head the axial meso- 
derm is divided into six somites. Transverse sections show that 





Epidermis ,, 




Somatic mesoderm / Proamnion ^\ > Splanchnic 
' \ mesoderm 



FIG. 202. 24 hour chick embryo. Transverse section through brain region. The 
neural folds have met but are not yet fused together. 


Axial mesoderm 




Vitelline vein Amnio-cardiac ~~ s P lanchn Pleure 


FIG. 203. 24 hour chick embryo. Transverse section through region of intestinal 

portal. X50. 

Neural groove 

Somite IE 


Intermediate mesoderm ,**.*. 



FIG. 204. 24 hour chick embryo. Transverse section through fourth somite. 



Primitive groove 



FIG. 205. 24 hour chick embryo. Transverse section through primitive streak. 



each has a minute cavity, or myocoel. The intermediate meso- 
derm does not divide into nephrotomes as in the frog. The 
lateral mesoderm is divided into the somatic and splanchnic 
layers. In the latter, numerous blood islands appear and give 
the characteristic mottled appearance to the area vasculosa. The 
coelom of the embryo is continuous with that of the extra-embry- 
onic regions, or exocoel. In the region on either side of the head, 
between the proamnion and the intestinal portal, the coelom is 
distended into an amniocardiac vesicle, so called because the so- 
matopleure will contribute to the head fold of the amnion, while 
the splanchnic inesoderm will give rise to the primordia of the 
heart, and the cavities of the vesicles will unite to form the 
pericardial cavity. The notochord, from its point of origin, the 
primitive streak, extends forward into the head. 

Ectodermal derivatives. The ectoderm at this stage con- 
sists of the elongate neural plate, with its groove and folds which 
are already in process of fusion, and the epidermis or non-nervous 


External form. In the chick embryo, after thirty-three hours' 
incubation, the length has increased to 4.3 mm. There is a 
slight bending of the head downward over the end of the noto- 
chord, foreshadowing the cranial flexure. The area vasculosa, 
in which the blood islands are being converted into capillaries, 
now has grown in toward the embryo, so that the area pellucida 
persists only around the head and tail regions. The anterior 
horns of the area vasculosa have met in front, completely in- 
closing the proamnion. The head has increased in length not 
only by actual forward growth but also by the backward extension 
of the lateral margins of the head fold, so that the enclosed fore- 
gut is now 1 mm. long. The vitelline veins are prominent at 
the margins of the intestinal portal and continue on the ventral 
side of the fore-gut to meet at the posterior end of the heart, 
which is now a single tube, slightly bent toward the right. The 
neural folds are fused as far back as the eleventh somite, where 
the posterior neuropore is now known as the rhomboidal sinus. 
The anterior neuropore is about to close, and in the head the 
neural tube shows three regions of dilation which represent the 



Head fold , 
of amnion 




" -VNy y \ Optic 
* '* ,1 vesicle 

1 i " ''* 

^^' ' 

^ Foregut 

,^ J l rhomboidialia 


FIG. 206. 33 hour chick embryo. Cleared preparation from dorsal view. X25. 



pf amnion 




Mesencephalon - 
Fore-gut - 

cavity I 

Rhombencephalon x 
Notochord " 

s Heart 

s Anterior 



fore-brain, mid-brain, and hind-brain, respectively. The sides 
of the fore-brain are evaginating to produce the optic vesicles. 

In the hind-brain, five neuromeres 
can be identified. Twelve somites 
may be counted. The notochord 
extends forward to the fore-brain 
from the primitive streak which is 
now reduced to 0.3 mm. 

Endodermal derivatives. The 
anterior end of the fore-gut is in 
contact ventrally with the stomo- 
deum separated only by the oral 
plate, composed of ectoderm and 
endoderm. At the sides, the walls 
of the fore-gut are fused to the ecto- 
derm at points where the first vis- 
ceral pouches (hyomandibular) will 
be located. 

Mesodermal derivatives. The 
somites now number twelve, and 
myocoels are still apparent. The 
mesomere is still unsegmented, but 
pronephric tubules have appeared 
in the region corresponding to so- 
mites 5-12. The four posterior 
tubules are growing back to form 
the pronephric duct. In the 
splanchnic mesoderm the blood 
islands are being converted into 
capillaries. The vitelline veins are 
prominent and continue forward 
into the heart, of which the endo- 
cardium and myocardium are dis- 
tinct. The heart is supported by 
the dorsal mesocardium, the ventral 
FIG. 207. 33 hour chick embryo. me socardium having disappeared. 

Sagittal section. X25. m i . ,. , , , ,, 

The primordial tubes, from the 

fusion of which the heart arose, continue forward as the ventral 
aortae which bend around the pharynx (first aortic arches) and 

- Primitive 


continue backward along the dorsal surface of the pharynx as the 
dorsal aortae. At the level of the primitive streak they are lost 
in a capillary nexus which foreshadows the vitelline arteries. 
From a point immediately in front of the optic vesicle, the anterior 
cardinals course backward on either side of the neural tube, bend- 
ing down ventrally to enter the heart with the vitelline veins. 
The notochord is slightly bent at the anterior end. 

Ectodermal derivatives. The neural folds now extend to 
the eleventh somite and have fused throughout the length of the 
head. The anterior neuropore is almost closed. The three 


Epidermis I Mesenchyme 

w Optic vesicle 

f\^ ,,... 

M^iV. * qr"'- T -v /<*v. 

*^'v S -^ 


Splanchnopleure Sub-cephalic 


FIG. 208. 33 hour chick embryo. Transverse section through optic vesicles. 


dilations which represent the prosencephalon, mesencephalon, 
and rhombencephalon are distinct. From the prosencephalon 
the two optic vesicles extend to the ectoderm of the sides of the 
head. Five neuromeres may be identified in the rhombencepha- 
lon. It has been asserted that in earlier stages three neuro- 
meres may be identified in the prosencephalon and two in the 
mesencephalon, while the first of the five noted above has re- 
sulted from the fusion of two original neuromeres destined to 
give rise to the metencephalon. At about this time a shallow 
depression^ in the floor of the prosencephalon, just in front of the 
tip of the notochord, marks the appearance of the infundibulum. 
The auditory placodes may sometimes be seen in sections as 
thickenings at the level of the constriction separating the last two 
neuromeres on either side. 








Otic ( auditory) placode 

, Dorsal aorta 
* Lateral sulcus 



Endocardium Splanchnopleure 

FIG. 209. 33 hour chick embryo. Transverse section through otic placodes. 


Spinal cord 

Dorsal aorta 
Lateral sulcus 


Intermediate mesoderm 

Coelom . ,,..,. 


Vitelline vein 

FIG. 210. 33 hour chick embryo. Transverse section through vitelline veins. 


Neural crest 
Dorsal aorta 

Spinal cord 


Intermediate mesoderm 
Somatic layer 

^^-^p---'^ | -W "*s ^ - 

Notochord Mid-gut 
FIG. 211. 33 hour chick embryo. Transverse section through sixth somite. X50. 



External form. The chick at the end of the second day of 
incubation has usually attained a length of 7 mm., but the form 
of the body has been altered profoundly. As the head has been 
lifted away from the blastoderm, it has increased greatly in size, 

Rhombencephalon ^/ ", V ,"' 
Otic vesicle 

Visceral cleft I 


Sinus venosus 
Vitelline vein 

Neural tube 


/ u <" . 

^U^ Optic cup 

Lens vesicle 

'" ;;- Prosencephalon 
Bulbus artenosus 


Amniotic fold 
Somite XDZ 

Vitelline artery 

Tail fold 


FIG. 212. 48 hour chick embryo. Transparent preparation from dorsal view 
(head from right side). X15. 

and the cranial flexure, which was just appearing in the thirty- 
three hour chick, has become so pronounced that the anterior 
end of the head is directed backwards. With this growth and 
flexure the head is twisted normally to the right, until it lies on 
one side, a phenomenon known as torsion. At forty-eight hours, 
this torsion involves the chick as far back as the seventeenth 
somite. The posterior end of the chick lies in its original position, 
and at the extreme caudal end a tail fold is be?,ng formed. In the 







\ IStomodaeal 


FIG. 213. 48 hour chick embryo. Head in sagittal section, somite region in 
(304) frontal section due to torsion. X50. 



area vasculosa the capillaries have formed attachments with the 
vitelline arteries and veins, and at the border of this area is a cir- 
cular vessel, the sinus terminalis. The fore-gut is now 1.4 mm. 
in length, and the first of the three visceral pouches now com- 
municates to the exterior following the rupture of the closing 
plate which separated it from the corresponding visceral groove. 
The second and third visceral grooves are apparent, but their clos- 
ing plates are still unperf orated. In the visceral arches the first 
three aortic arches are apparent, arising from the ventral aorta. 
The heart is now twisted so that the ventricular loop is upper- 

Anterior cardinal vein 

Dorsal aorta 

Otic pit 


Aortic arch I 
Lens pit 
Optic cup 

Yolk sac Notochord 

Visceral groove I 

Blood island 
Pigment layer 

Visceral pouch I Hypophysis Sensory layer 

FIG. 214. 48 hour chick embryo. Transverse section through otic pit and optic 

cup. X50. 

most. The vitelline veins are large and conspicuous, as are the 
vitelline arteries which leave the body at the level of the twenty- 
second somites. The neural tube is completely closed. In the 
head the five definitive regions of the brain are outlined, the 
prosencephalon having given rise to the telencephalon and dien- 
cephalon, and the rhombencephalon to the metencephalon and 
myelencephalon. The eye is now in the optic cup stage, and the 
invagination of the optic vesicle continues down the stalk to form 
the choroid fissure. The lens is in the form of a pit which has 
almost attained the vesicle stage. The ear is represented by an 
otic pit which, owing to the cervical flexure, is about on a level 
with the eye. There are twenty-seven somites at this stage. 
The primitive streak is found only in the tail fold. At this time 


the head fold of the amnion has grown back over the chick as 
far as the sixteenth somite. 

Endodermal derivatives. The stomodeum, an ectodermal 
invagination from the ventral surface of the head fold, has formed 
the oral membrane by contact with the fore-gut a little back of its 
most anterior point. Hence there is a blind pocket in front of 
the oral plate, known as the preoral gut. Three visceral pouches 
are present, the first of which opens into the corresponding visceral 
furrow following the rupture of its closing membrane. The 
primordium of the thyroid is represented by a ventral depression 
in the floor of the pharynx at the level of the second visceral 
pouches. The primordia of the lungs (sometimes difficult to 
distinguish) extend to the level of the sinus venosus. The liver 
arises at the level of the anterior intestinal portal from two 
evaginations of the endoderm, one below and one above the 
meatus venosus. The mid-gut now has two shifting boundaries, 
the anterior intestinal portal and the posterior intestinal portal. 
The latter is barely apparent as the opening of a shallow endo- 
dermal pocket or hind-gut in the tail fold. 

Mesodermal derivatives. The somites, twenty-seven in 
number, show a varying degree of specialization, with the most 
advanced at the anterior end. In these two regions can be dis- 
tinguished : a loose aggregate of cells at the median ventral angle 
(the sclerotome) ; and a cap of epithelial cells at the lateral dorsal 
angle. The cells of this cap nearest the epidermis will form the 
dermatome, while those nearest the neural tube will form the 
my o tome. 

The pronephric tubules in the more anterior somites have dis- 
appeared and mesonephric tubules are appearing in the meso- 
mere posterior to the thirteenth somite. The pronephric (now 
the mesonephric) duct has acquired a lumen but has not yet 
attained its complete backward growth. 

The heart is still tubular, but the ventricular limb of the cardiac 
loop has grown back and over the atrial limb so that the ven- 
tricular region is now caudal and dorsal with relation to the 
atrial region. Three aortic arches are present as a rule, but in- 
frequently the third has not developed. From the first aortic arch 
a network of capillaries extends into the head. From these the 
carotid arteries will be formed. The dorsal aortae have fused 



from a point back of the sixth somite as far as the level of the 
fifteenth somite. The vitelline arteries leave the dorsal aortae at 
the level of the twenty-second somite but the aortae continue 

Dorsal aorta 

Spinal cord 
Epidermis > 

Common cardinal Bulbus 

vein arteriosus chorion 


Yolk sac 


'Dorsal mesocardium 
FIG. 215. 48 hour chick embryo. Transverse section through heart. X50. 

backward as the caudal arteries to the last somite. The vitelline 
veins are fused at their point of entrance into the heart as the sinus 
venosus. The anterior cardinals are prominent and extend from 
a capillary plexus in the head back toward the heart, where they 



Notochord | Dorsal aorta 

Vitelline vein, 

*.,-, Ventricle 

; . <m 


Posterior cardinal 

FIG. 216. 48 hour chick embryo. Transverse section through liver. X50. 

are joined by the posterior cardinals and proceed as the common 
cardinals tol enter the heart in the angles between the sinus 
venosus and the vitelline veins. The posterior cardinals may be 
traced back to the last somite. The heart of the chick commenced 



beating at the forty-fourth hour of incubation, so that the course 
of the blood is through the ventral aorta to the aortic arches and 
thence to the dorsal aorta. From the first aortic arch a network 
of capillaries supplies the head with blood (which is returned by 
way of the anterior cardinals). The main current of the stream 
passes down the dorsal aortae to the point where these fuse to 
form the median dorsal aorta. From the dorsal aorta, the somites 
are supplied by capillaries, which will later become the interseg- 
mental arteries. This blood is returned through the posterior 
cardinals. Leaving the dorsal aorta by way of the vitelline 
arteries, the blood passes through the capillaries of the area 
vasculosa to the sinus terminalis, and thence to the capillary 
drainage of the vitelline veins which return it to the heart. 

The notochord is bent, not only at its tip (cranial flexure) but 
also at the point where the myelencephalon merges with the spinal 
cord (cervical flexure). 

Ectodermal derivatives. The brain now has acquired its 
five definitive vesicles. The telencephalon is enlarged but shows 

Amniotic raphe 

Spinal cord 



Posterior cardinal vein 
Mesonephric tubules 


CoelonT -^^ " Mid-gut rSa 

Lateral sulcus 

FIG. 217. 48 hour chick embryo. Transverse section through mesonephros. 


no particular differentiation. From the diencephalon project the 
constricted optic stalks which bear the optic cups with their inner 
sensory layer and outer pigmented layer. (The pigment will 
not arise until later.) The invagination by which the cups were 
formed continues down the stalk as the choroid groove. On the 
ventral surface of the diencephalon the infundibulum has deep- 
ened. Growing in toward it from the stomodeum is an ecto- 
dermal invagination, the hypophysis, which will fuse with the 
infundibulum to form the pituitary gland. The lens of the eye 



is in the pit stage, resulting from the invagination of a sensory 
placode. When the process is complete, the lens will be a vesicle 
completely withdrawn beneath the surface of the ectoderm, as 
will the otic vesicle, the primordium of the inner ear. Along the 
rhombencephalon and cord, the neural crest is to be seen as a 
narrow band of cells on each dorso-lateral angle. 



Otic vesicle 

cleft I 

Choroid fissure 

.Optic cup 
and lens 





Somite 26 

limb bud 

Fia. 218. 72 hour chick embryo. Transparent preparation from dorsal view, 
head seen from right side. X15. 


External form. At the end of the third day of incubation, 
the total length of the embryo is 9.5 mm., but the curvature of 
the body is so great, on account of the cranial and cervical flex- 
ures in addition to the newly developed caudal flexure, that the 
greatest length, from neck to tail, is 7 mm. Torsion involves the 


body as far back as the vitelline arteries and will become com- 
plete during the fourth day. Anterior and posterior limb buds 
are now apparent at the levels of somites 17-19 and 26-32 re- 
spectively. The tail is curved forward. The fore-gut is still 
1.4 mm. in length but has undergone further differentiation, 
indicated externally by the fact that the first three visceral 
clefts are open while the fourth is still interrupted by its closing 
plate. In the branchial arches four aortic arches may be seen. 
The telencephalon has given rise to the primordia of the cerebral 
hemispheres, and from the roof of the dienccphalon, a small 
evagination represents the epiphysis or primordium of the pineal 
gland. The eye and ear, which were formerly in the same 
transverse section, are now nearly in an antero-posterior relation- 
ship. The olfactory pits have made their appearance in the 
head. The semilunar (fifth cranial nerve), geniculo-acoustic 
(seventh and eighth), and petrosal (ninth) ganglia may be seen. 
There are approximately thirty-five somites. The primitive 
streak has disappeared. The aninion is completed by the fusion 
of head and tail folds. The allantois, a small sac-like evagination, 
protrudes ventrally between the posterior limb buds. 

Endodermal derivatives. At the end of the third day the oral 
aperture has been formed by the rupture of the oral membrane 
separating the stomodeum and the fore-gut. Immediately an- 
terior to this opening the preoral gut persists. The fore-gut is 
still the same length as in the chick of forty-eight hours, but is 
more complex in structure. The thyroid gland, which appeared 
during the second day, has now become differentiated into the 
distal dilation which will give rise to the gland proper and the 
thyroglossal duct. The first three visceral pouches are open 
to the exterior, but the epithelial buds destined to give rise to the 
thymus and parathyroids are not yet apparent. The fourth 
visceral pouch is still separated from the corresponding groove by 
the closing plate. The laryngeo-tracheal groove has developed 
in the floor of the pharynx just posterior to the fourth visceral 
pouches. At its posterior end the dorsal margins of this groove 
have closed together to form the primordium of the trachea which 
is thus set free from the esophagus above. The trachea is bi- 
furcated at the posterior end, thus giving rise to the two bronchial 
buds which are the primordia of the lungs. 



The esophagus, which is relatively narrow, is followed by a 
dilation which is to become the stomach. Posterior to this, the 
primordium of the liver may be seen as an evagination from the 



Aortic arches 








Spinal cord 

FIG. 219. 72 hour chick embryo. Sagittal section. X25. 

ventral floor of the duodenal region of the gut. The dorsal 
pancreas arises from the duodenal region just dorsal to the liver 
at the end of the third day. The ventral primordia will not 
appear for another day. 



Otic vesicle: Ganglion 



"Yolk sac 

Anterior cardinal vein 
FIG. 220. 72 hour chick embryo. Transverse section through otic vesicle. X25. 

Dorsal aorta 

Lens vesicle 


Sensory layer 

Pigment layer 




Anterior cardinal vein 



Visceral arches 


FIG. 221. 72 hour chick embryo. Transverse section through optic cup. X25. 

Esophagus prim Common cardinal Bulbus arteriosus 

inn A mm' nn i , / Vein S ' ^ ,-.- /r:vv> *>-. 

Chorion Amnion 


Dorsal aorta 

Pleural groove , 

'elencephaloriJv'i^VTVf/ Epidermis 


Yolk sac 

Atrium Nasal pit 
'venosus Pericardial 

FIG. 222. 72 hour chick embryo. Transverse section through heart and lung. 




The mid-gut region is gradually lessened by the advancing 
sulci which are cutting off the body of the embryo from the yolk. 
This region opens into the yolk stalk which is still quite wide. 

The hind-gut contained in the tail fold has not yet acquired 
its cloacal aperture nor has the proctodeum appeared. The 
floor of the hind-gut between the tail bud and the posterior 
intestinal portal evaginates to give rise to the allantoic pri- 

Mesodermal derivatives. The somites, typically thirty-five 
in number, still show a varying degree of differentiation which is 
carried to its furthest point in the more anterior somites. The 
dermatome is now a thin sheet of cells along the dorso-lateral 


Dorsal cardinal Dorsal 
Amnion aorta vpin mesentery 





Allantoic vein 



Meatus venosus 

^Ventral mesentery 
FIG. 223. 72 hour chick embryo. Transverse section through liver. X25. 

angle of the embryo, with the myotome parallel and internal; 
the sclerotome in these anterior segments is a large and loose 
aggregate of cells investing the neural tube, notochord, and 

The pronephric tubules have degenerated to a considerable 
extent, but the nephrostomes opening into the coelom may per- 
sist. The mesonephric tubules are now in process of develop- 
ment, with those in the more anterior segments most highly 
differentiated. The tubules between the thirteenth and thir- 
tieth somites have progressed from the vesicle stage character- 
istic of those behind the twentieth somite, and some have ac- 
quired a lumen and joined the pronephric duct which hence- 
forward is known as the mesonephric duct. A few of the more 
anterior tubules develop nephrostomes, but these soon disappear. 



Behind the twentieth somite, as far back as the thirtieth, only 
vesicles are formed. The mesonephric ducts have grown back 
and united with the cloaca. 

The heart now shows a constriction between the atrial and 
ventricular region. Four aortic arches are developed, of which 


Dermatomev Scler P tome \ Spinal cord 





/ Mid-gut 

Lateral sulcus Dorsal 



FIG. 224. 72 hour chick embryo. Transverse section through vitelline arteries 

leaving body. X25. 

the first is becoming smaller, and sometimes has disappeared at 
this stage. The internal carotid arteries are now well developed, 
growing forward into the head from the point of union between 
the first arches and the dorsal aortae. From the ventral end of 
the first aortic arch the external carotid takes its origin. The 

Chorion . Mesonephric 
Amnion \ duct Somite 
Leg bud , 

Dorsal aorta 

, Hind-gut 

FIG. 225. 72 hour chick embryo. Transverse section through allantois. X25. 

pulmonary is sometimes apparent as a posterior prolongation of 
the ventral aorta at the point where the fifth arches will appear 
during the next twenty-four hours. The intersegmental arteries 
are now apparent as dorsal diverticula from the aorta between 
each pair of somites. The vitelline veins have fused for a short 
distance behind the sinus, thus giving rise to the meatus venosus. 


The anterior cardinal vein now possesses many branches from 
the head, among which are three intersegmental veins. The 
posterior cardinal has continued its backward growth dorsal 
to the mesonephric duct as far as the thirty-third somite. It 
receives the intersegmental veins of this region. Where the 
posterior cardinals unite with the common cardinals, a capillary 
network indicates the beginnings of the allantoic veins. 

Ectodermal derivatives. The brain at the end of the third 
day has its five definitive vesicles even more sharply demarcated. 
From the telencephalon two lateral vesicles have evaginated to 
form the primordia of the cerebral hemispheres. In the dien- 
cephalon the epiphysis has appeared as a dorsal evagination. 
On the floor of this vesicle the infundibulum is almost in contact 
with the hypophysis. The mesencephalon is separated from the 
metencephalon by a deep constriction known as the isthmus. 
Along the sides of the myelencephalon may be distinguished the 
following cerebral ganglia: the semilunar of the fifth cranial 
nerve; the acoustico-facialis which will later separate into the 
geniculate ganglion of the seventh and the acoustic of the eighth; 
and the petrosal ganglion of the ninth. The eye has increased in 
size, and the lens is now free from the epidermal ectoderm. The 
ear, too, is in the vesicle stage and possesses a short endolym- 
phatic duct, which has lost its connection with the epidermis. 
On the third day the primordium of the nose is represented by 
two olfactory pits anterior to the mouth. 


Arey, L. B. 1934. Developmental Anatomy, 3rd Ed., Chap. 18. 
Duval, M. 1889. Atlas d'ernbryologie. 

Keibel and Abraham. 1900. Norrnaltafeln II, dcs Huhnes (Callus domesticus). 
Lillie, F. R. 1919. The Development of the Chick, 2nd Ed. 
,McEwen, R. S. 1931. Vertebrate Embryology, 2nd Ed., Part 4. 
Patten, B. M. 1929. The Early Embryology of the Chick, 3rd Ed. 


Pig embryos of 10 to 12 mm. body length are particularly 
instructive for laboratory work in mammalian embryology as they 







limb bud 

Roots of 
spinal nerves 


Body stalk 

V,'/ limb bud 


FIG. 226. 10 mm. pig embryo. Transparent preparation from right side. Xll. 

are sufficiently large for the study of external structures and yet 
small enough to afford serial sections for a detailed study of the 
anatomy. The primordia of practically all the organ systems, 
excepting the skeleton and musculature, are present. In com- 
paring the accounts given by different authors of this particular 
stage, it should be remembered that a large amount of shrinkage 
takes place during the preparation of fresh sections, so that, as 



pointed out by Patten, an embryo of 12 mm. will not measure 
more than 9J mm. when prepared for sectioning. The account 
which follows corresponds in general to the pig (Sus scrofa) of 
10 mm. described by Keibel, of 12 mm. (Minot), 10 mm. (Prentiss) 
and 9.4 mm. (Patten), but is not so advanced as the 13.5 mm. 
pig (Boyden). 

External form. The pig embryo at this stage is relatively 
/more advanced than the chick of seventy-two hours. The body 
is sharply flexed, owing to the presence of the cranial, cervical, 
dorsal, and caudal flexures. In the head region the olfactory 
pits are well developed and are connected by the naso-lachrymal 
groove to a depression which surrounds the bulging eyeball. 
The five divisions of the brain are apparent through the rela- 
tively thin overlying epidermis. Four visceral grooves can be 
seen, the first of which, or hyomandibular, is the primordium of 
the external auditory meatus. The third and fourth grooves are 
compressed by the cervical flexure into a deeper depression known 
as the cervical sinus. A frontal view of the head shows the oral 
cavity bounded above by the frontal process in the middle, the 
maxillary processes at the side, while the lower jaw is represented 
by the mandibular arch. 

In the trunk region, the buds of the pectoral and pelvic ap- 
pendages are large but show no further differentiation. The 
contours of the somites, now forty-four in number, are apparent 
along the back, and ventral to these can be seen the outlines of the 
heart, liver, and mesonephros. In some specimens there appears 
between the limb buds a thickened ridge from which the mammary 
glands develop and which is therefore known as the milk line. 
The umbilical cord projects from the ventral side of the embryo. 
Between this and the base of the slender tail is a small pro- 
tuberance, the genital tubercle, or primordium of the external 

Endodermal derivatives. The preoral gut still persists an- 
terior to the oral aperture. Ventral to this, and seen best in 
sagittal section, is the long and slender hypophysis, now in con- 
tact with the infundibulum of the diencephalon. Both the 
hypophysis and infundibulum, it should be remembered, are of 
ectodermal origin. The pharynx is dorso-ventrally compressed, 
and from its floor the tongue is arising. Four visceral pouches 



are present, corresponding to the visceral grooves already noted. 
These do not unite to become visceral clefts but remain separated 
by their closing membranes. Between the second and third 




Aortic arch 



Pre oral 



iptic chiasma 



(ant. mesenteric 




Ductus venosus 


Duct of ventral pancreas 
Duodenum Vitelline vein 





FIG. 227. 10 mm. pig embryo. Sagittal section. X16J. 

pouches the thyroid gland appears. From the level of the fourth 
pouch a short laryngeal groove is prolonged into the trachea 
which has given rise to the bronchial buds, three in number. Two 
of these, the primary bronchi, have arisen by the bifurcation of 



the trachea; the third or apical bud, which will give rise to the 
eparterial bronchus, develops anterior to the right primary 
bronchus. The esophagus is relatively long and narrow and, 
just posterior to the level of the lung buds, passes into the stomach 
which is dilated and shows a slight dorsal curvature. Posterior 
to the stomach the duodenal glands, liver, and pancreas are well 
developed. The liver, now a large glandular mass traversed by 

Nerve XI 


and jugular " ff 



Ganglion Y 

carotid artery 

Ganglion IX 


Otic vesicle 

Ganglion v 


Nerve III 

FIG. 228. 10 mm. pig embryo. Transverse section through otic vesicles. X16J. 

the capillaries of the hepato-portal veins, retains its original con- 
nection with the duodenum as the common bile duct from the 
distal end of which the gall bladder is forming. Both dorsal 
and ventral primordia of the pancreas are present, the duct of 
the latter arising from the common bile duct. The long and 
slender intestine extends into the umbilical cord as the intestinal 
loop, to which the yolk stalk is still attached. Just posterior to 
this, a slight enlargement may sometimes be observed which in- 
dicates the boundary between the large and small intestine. The 
hind-gut is dividing into a dorsal rectum and ventral urogenital 



sinus, prolonged into the allantoic stalk. The sinus and rectum 
unite in a common cloaca which has not yet established connection 
with the proctodeum. Immediately posterior to the cloacal plate, 
a small blind pocket represents the postcloacal gut. 

Spinal cord. 


cardinal vein 



Optic cup 

Dorsal root 
Spinal ganglion 
Ventral root 
Dorsal ramus 

Ganglion X 
( nodosum ) 
Radix aortae 

Visceral arch 



cardinal vein 

Sensory layer 
Pigment layer 


HH li l""~ 

FIG. 229. 10 mm. pig embryo. Transverse section through optic cup. X16J. 

Mesodermal derivatives. The notochord extends from the 
vicinity of the floor of the mesencephalon into the tail, following 
the flexures of the body. 

The somites have long since become differentiated into the 
myotome, dermatome, and sclerotome. In the tail region, the 
sclerotomes are separated into the cranial and caudal arcualia 
from which the vertebrae will originate. 

In the pig of 10 mm., the pronephric stage has been passed; 
the mesonephros is at the height of its development, forming a 
great "Wolffian" body with a complicated network of interwoven 
tubules; while the mesonephric duct (originally the pronephric 
duct) may be recognized along the ventral margin. Emerging 



from the mesonephros, each duct enters the urogenital sinus at the 
same level as the allantoic stalk. From each duct a narrow stalk 
runs dorsally and forward as the metanephric duct, or ureter, 
which at its distal end is enlarged to form the pelvis of the meta- 
nephros. Around the pelvis the posterior portion of the nephro- 
tomal band will produce the secretory tubules of the definitive 
kidney at a later stage. On the median ventral margin of each 

Spinal cord 






Dorsal aorta 

cardinal vein 



FIG. 230. 

10 mm. pig embryo. Transverse section through nasal (olfactory) pit. 

mesonephros are slight swellings which will later become the 
genital ridges, primordia of the gonads. The coelom is partially 
divided into the pericardial and abdominal cavities by the septum 
transversum. The mesenteries of the principal viscera are in 
evidence. The liver is still suspended in the ventral mesentery. 
A dorsal mesocardium is present. 

The heart of the 10 mm. pig has the four main chambers estab- 
lished, although not yet completely separated into right and left 
halves. The sinus venosus now enters the right atrium through 



a slit guarded by the valves of the sinus. The right and left 
atria are partially separated by the interatrial septum in which 
can be seen an opening, the foramen ovale. The atrio-ventric- 
ular canal leading to the ventricle is partially separated into right 
and left halves by the endocardial cushion. The ventricle is par- 
tially divided by the interventricular septum. From the ventral 
aorta three aortic arches curve around the pharynx to unite with 
the dorsal aorta. These are the third, fourth, and sixth aortic 
arches; the first and second have degenerated, while the fifth 

Spinal cord 


cardinal vein 

Valves of 



limb bud 

Dorsal aorta 


Left atrium 


FIG. 231. 10 mm. pig embryo. Transverse section through sinus venosus. X 16 J. 

seldom appears as a separate structure. The pulmonary arteries 
are growing back from the sixth aortic arches. 

As prolongations of the original paired ventral and dorsal aortae, 
the external and internal carotid arteries, respectively, run for- 
ward into the head. The internal carotid arteries are united at 
the level of the isthmus between the mesencephalon and the 
metencephalon with the basilar artery, which serves to unite 
them with the vertebral arteries, arising from the anastomosis of 
intersegmental arteries in the cervical region. At the 10 mm. 
stage the vertebral arteries have lost their intersegmental con- 
nections with the aorta except at the posterior end, where the 



Spinal cord, 


limb bud- 


vena cava 




Dorsal aorta 

Cardinal vein 

Meson ephr os 


Lung bud 


FIG. 232. 10 mm. pig embryo. Transverse section through lung buds. 

Spinal cord 


vena cava 



Dorsal aorta 




FIQ. 233. 10 mm. pig embryo. Transverse section through stomach. X16J. 



seventh cervical intersegmental artery persists and grows out into 
the pectoral limb bud to form the subclavian artery. Near the 
point of origin of the subclavian, the dorsal aortae are fused and 
run back as a single median aorta into the tail. Dorsally, 
branches are given off from the aorta as intersegmental arteries 
of the trunk. Laterally, many small branches supply the glo- 
meruli of the mesonephros. Ventrally, the dorsal aorta gives 
off the coeliac artery and anterior mesenteric arteries to the gut. 

Spinal cord 


Dorsal aorta 


Portal vein 





Left umbilical 

'Body stalk 

artery ; 


FIG. 234. 10 mm. pig embryo. Transverse section through gall bladder. Xl6f. 

Two large umbilical (allantoic) arteries run from the dorsal aorta 
into the umbilical cord. The aorta continues into the tail as a 
relatively slender vessel, the caudal artery. 

The vitelline veins are much smaller than in the chick of 
seventy-two hours, for the yolk sac from which they drew their 
blood is nearly degenerated. In the pig at this stage they drain 
the gut area and cross into the liver where they become the portal 
vein. Within the liver they are broken up into capillaries which 
emerge as the hepatic veins to the sinus venosus. Of the somatic 



veins, the anterior cardinals are still prominent and are joined 
by an extensive series of head veins. In the cervical region the 
anterior cardinals receive the dorsal intersegmental veins as well 
as the external jugular from the mandible. As the anterior 
cardinals enter the common cardinal veins, they are joined by the 
posterior cardinals, which have already lost part of their drainage 

Spinal cord 

Dorsal aoi 



FIG. 235. 
















"/.I..JAJ.. / , r 7?7T^*f '!.*9uFV >!>jf 

_ . . - - ^mr J> ,<>* 7(a&%? tt ' y 
artery f 




10 mm. pig embryo. Transverse section through umbilical stalk in 
region of intestinal loop. X16J. 





area to the subcardinal veins passing through the ventral portions 
of the mesonephroi. Numerous small venous channels serve to 
connect the subcardinals arid postcardinals during this period. 
The posterior caval vein has already made its appearance as a 
direct connection from the subcardinals to the liver. The um- 
bilical (allantoic) veins proceeding from the allantois toward the 
heart are fused together in the umbilical cord. In the body they 



pass through the liver, within which they are, like the vitelline 
veins, broken up into capillaries. The left umbilical maintains a 
broad channel through the liver. This vessel, now known as the 
ductus venosus, connects the umbilical with the posterior caval 

Spinal cords 


Dorsal aorta 




limb bud 




FIG. 236. 10 mm. pig embryo. Transverse section through metanephric duct and 
posterior limb buds. X 16 J. 

Ectodermal derivatives. The epidermal derivatives of the 
ectoderm have already been enumerated in the description of 
external form. There remain for consideration the nervous sys- 
tem and sense organs. The five definitive vesicles of the brain 
are well marked. From the telencephalon arise the two lateral 
cerebral vesicles. This division of the brain is separated from 
the diencephalon by two points of reference, the optic recess in 
the floor, and the velum trans versum in the roof. From the 
diencephalon spring the optic stalks, leading to the optic cups, 
and the infundibulum, now in contact with the hypophysis as 
mentioned above. The posterior boundary of the diencephalon 
is indicated by the tuberculum posterius arising from the brain 
floor. The epiphysis seldom appears at this stage. The mesen- 
cephalon, with the third cranial nerve arising from its floor, is 



demarcated at its posterior end by the deep constriction of the 
isthmus. The metencephalon is distinguished from the myelen- 
cephalon by its thicker roof. From the isthmus the fourth 
cranial nerve runs forward laterally over the sides of the brain to 
the mass of mesoderm surrounding the eyeball, from which the 

Neural tube 








carotid artery 

Olfactory pit 
3rd Aortic arch 
4th Aortic arch 
6th Aortic arch 





Neural tube- 

FIG. 237. 10 mm. pig embryo. Frontal section through aortic arches and ductus 

venosus. X16J. 

eyeball muscles will be formed. Conspicuous at the anterior 
ventro-lateral margin of the metencephalon is the large semilunar 
ganglion of the fifth cranial nerve. From the floor of the myelen- 
cephalon, the sixth cranial nerve emerges to run forward toward 
the eye. Immediately following this, the geniculate ganglion of 
the seventh and the acoustic ganglion of the eighth are in close 


connection. The ninth cranial nerve has two ganglia, the dorsal 
superior ganglion and ventral petrosal, while the tenth similarly 
possesses a dorsal jugular and ventral nodose ganglion. The 
eleventh cranial nerve possesses at this stage a small ganglion 
(of Froriep) which disappears in the adult. The last of the cranial 
nerves, the twelfth, arises from the floor of the myelencephalon 
by a number of small roots and without a ganglion. In the region 
of the spinal cord the segmental nerves arise from the cord by two 
roots, of which the dorsal is associated with a spinal ganglion. 
The trunk is very short and soon divides into three main branches. 
The dorsal and ventral rami run to these respective regions of 
the body wall, while the third, or communicating ramus, unites 
the spinal nerve with a ganglion of the sympathetic chain. The 
sympathetic ganglia may be recognized as small masses of cells 
dorsal to the aorta. 

The nose is represented by the olfactory pits. The eye is in 
the optic cup stage with a well-marked choroid fissure and 
groove, while the lens is completely separated from the outer ecto- 
derm and is in the vesicle stage. Of the various regions of the 
ear, all the primordia are now established. The otic vesicle with 
its endolymphatic duct, representing the inner ear, is in close 
juxtaposition to the first visceral pouch (hyomandibular) which 
will give rise to the auditory tube and chamber of the middle ear; 
the external auditory meatus, or outer ear, will arise from the 
first or hyomandibular groove. 


Arey, L. B. 1934. Developmental Anatomy, 3rd Ed., Chap. 19. 
xBoyden, E. A. 1933. A Laboratory Atlas of the Pig Embryo. 
Keibel, F. 1897. Normaltafeln, I, des Schweines (Sus scrofa domesticus) . 
Lewis, F. T. 1902. The gross anatomy of a 12 mm. pig, Am. Jour. Anat, Vol. 2, 

pp. 211-226. 

Minot, C. S. 1911. A Laboratory Textbook of Embryology, 2nd Ed. 
Patten, B. M. 1931. The Embryology of the Pig, 2nd Ed. 
Wallin, E. 1917. A teaching model of a 10 mm. pig embryo, Anat. Rec., Vol. 5, 

pp. 17-45. 



A method much employed in the study of comparative em- 
bryology is that of cutting a preserved egg or embryo into a series 
of extremely thin slices, and arranging these in order upon a 
glass slide, so that they may be examined under the microscope. 
The older embryologists, however, were limited to the study of 
entire embryos and of minute dissections. These methods are 
still of great value in supplementing the study of serial sections, 
for it is a difficult mental exercise to translate sections into terms 
of the whole embryo. The single section, especially, is meaning- 
less except when interpreted as a part of the complete series. It 
is very helpful, therefore, when facilities permit, for each student 
to prepare for himself a whole mount and a series of sections 
through one of the embryos he is to study. 


Although preserved embryos of the more important laboratory 
types may be obtained from the biological supply houses, it is 
often desirable to collect and rear live embryos. 

THE FROG. There are some sixty species of tailless Amphibia 
within the continental limits of the United States. Although 
the capture of adults in a pond where eggs are found is strong 
circumstantial evidence as to the species of the eggs, even this 
evidence is often lacking, so that the ability to identify the 
eggs or larvae from their own characteristics is highly desirable. 
A key to the eggs and larvae of some of the common Eastern frogs 
and toads is found in Wright's " Life History of the Anura of 
Ithaca, N. Y." For the Pacific slope fauna, see Storer, " A 
Synopsis of the Amphibia of California." The eggs of the sala- 
mander, Ambystoma, are laid at the same time and in the same 
localities as those of the early frogs, but may be distinguished 
from them by the greater proportion of jelly to the eggs in the 
mass of spawn. 



Experiments dealing with the effect of pituitary hormones have 
led to the discovery that one of these hormones will induce 
ovulation in the female frog, and the drive to amplexus in the 
male, out of the breeding season. Hugh 1 (1934) has described in 
detail a technique for inducing ovulation and bringing about 
artificial fertilization which has been since used in several labora- 
tories, including the author's, with complete success. 

The rate of development of the frog's egg depends upon the 
temperature of the water. In the laboratory, the eggs will hatch 
in about one week after laying, at the ordinary room tempera- 
ture. The egg masses should be kept in clean glass containers 
with at least ten times as much water. The water should not be 
changed until after hatching, when the larvae should be trans- 
ferred to fresh water with aquatic plants. After the assumption 
of the tadpole form, they should be fed small pieces of finely 
ground meat. Metamorphosis may be hastened by feeding fresh 
or desiccated thyroid tissue. 

Artificial fertilization is the best method of obtaining the 
earliest stages of development. The testes and vasa defcreritia 
of the male are teased out in a watch glass of water. The eggs 
from the distal portions of the oviducts are placed in this water for 
five minutes and then removed to glass containers with not more 
than four inches of water. 

THE CHICK. In collecting hens' eggs for incubation, it is a 
truism that they must be fresh and fertile. The best results are 
obtained from trap-nested eggs in the spring semester. The egg 
is normally laid in the gastrula stage (Chapter II), but in those 
cases where the egg does not reach the distal end of the oviduct 
by 4 P.M., it is retained till the following morning and undergoes 
further development. After laying, the egg cools and develop- 
ment ceases until incubation is commenced. The fertilized egg 
is viable for five weeks at a temperature of 8-10 C. The time 
of hatching, as in the frog's egg, is dependent upon the tem- 
perature. The minimum temperature at which development will 
take place is about 25 C.; the optimum is 37 C., at which 
temperature the egg will hatch in twenty-one days; the maximum 
temperature is about 41 C. In incubating eggs, care must be 

1 R. Rugh. Induced Ovulation and Artificial Fertilization in the Frog, Biol. 
Bull. 66, 22-29. 


taken to keep the air in the incubator moist and to rotate the 
eggs once a day. 

Instructive demonstrations may be made by opening the shell 
and shell membranes under aseptic conditions and removing a 
bit of the albumen. A window of celloidiri placed over the open- 
ing and carefully sealed will permit of observations on the devel- 
opment of the embryo for several days. An alternative method 
is that of opening the egg and placing the contents in a sterilized 
small s tender dish. A glass ring is placed on the yolk to keep it 
beneath the surface of the albumen, and the dish is covered and 
placed in the incubator. If this operation is carried on under 
aseptic conditions, development will continue for two or three days. 

THE PIG. The early stages of development in any mammal 
are valuable. The larger embryos are visible as protuberances 
on the inner side of the uterine tubes. The tube should be slit 
open and the embryos exposed by cutting open the embryonic 
membranes which surround them. Smaller stages are obtained 
by washing out the contents of the tube with normal salt solution 
or preserving it entire. 

Pig embryos may be obtained in quantities from any good-sized 
packing house. As many as eighteen may be found in a single 
female, but the average number is eight. The period of gestation 
in the pig is 121 days. Pig embryos of 10 mm. body length are 
the most useful in the elementary course. Later stages are of 
value in the detailed study of organogeny. 


The preliminary preparation of material for microscopical work 
involves three distinct operations: killing, fixing, and preserva- 
tion. In practice, two or three of these operations are performed 
by a single reagent known as a " fixing fluid. " Such a reagent 
should kill the embryo so rapidly that it will undergo the minimum 
of post-mortem changes; it should preserve the structures of the 
embryo with as life-like an appearance as possible ; and it should 
harden the soft parts so that they may undergo the later processes 
of technique without loss of form or structure. Some fixing fluids, 
such as alcohol or formalin, may be used indefinitely as preserva- 
tives, but the majority are used for a particular optimum period, 
and then washed out and replaced by alcohol. 


THE FROG. The frog's egg, before hatching, is best fixed by 
Smith's fluid. 

Potassium bichromate 0.5 gram 

Glacial acetic acid 2.5 cc. 

Formalin 10.0 cc. 

Distilled water 75.0 cc. 

1. Cut the egg masses into small pieces of about twenty-five 
eggs each, and submerge them in a dish of Smith's fluid for 
twenty-four hours. A quantity equal to ten times the volume 
of the eggs should be used. 

2. Rinse the eggs in water and wash with a 5 per cent aqueous 
solution of formalin until no more free color comes out. The 
eggs may be kept indefinitely in this fluid. If it is desired to 
remove the egg membranes, proceed as follows : 

3. Wash in water for twenty-four hours, changing the water 
several times. 

4. Place the eggs in eau de Javelle, diluted with three time its 
volume of water, and shake gently from time to time during a 
period of 15 to 30 minutes until the membranes are almost 
dissolved and will shake off. 

5. Rinse in water and run through 50 per cent and 70 per cent 
alcohol, an hour to a day each, and preserve in 80 per cent alcohol. 

After hatching, larvae are best fixed in Bouin's fluid. 

Picric acid, saturated aqueous solution 75 cc. 

Formalin 25 cc. 

Glacial acetic acid 5 cc. 

1. Larvae are left in this fluid from one to eighteen hours, 
according to size. 

2. After rinsing in 50 per cent alcohol, wash in 70 per cent 
alcohol, to which has been added a few drops of lithium car- 
bonate, saturated aqueous solution, until the yellow color is 
extracted, and preserve in 80 per cent alcohol. 

THE CHICK. The chick embryo must be removed from the 
shell, albumen, and yolk before fixation. As the early stages 
are more difficult to handle, it is advisable to practice this opera- 
tion on embryos of seventy-two hours' incubation and then work 
backward toward the stages of the first day. 


1. Place the egg in a dish 3 inches high and 6 inches in diam- 
eter, two-thirds full of normal saline solution, warmed to 40 C. 

Sodium chloride 0.75 gr. 

Water 100.00 cc. 

2. Crack the shell at the broad end with the flat of the scalpel, 
and pick away the pieces of shell until an opening slightly larger 
than a half dollar has been made. Remove the outer and inner 
shell membranes. Invert egg beneath the surface of the salt 
solution and allow the contents to flow out. The blastoderm, 
containing the embryo, will rotate until it is uppermost. With 
fine-pointed scissors, cut rapidly a circle of blastoderm, about the 
size of a quarter, with the embryo at the center. With blunted 
forceps, pull the blastoderm and adherent vitelline membrane 
away from the yolk and albumen, waving it gently beneath the 
surface of the salt solution to remove all yolk. 

3. Submerge a Syracuse watch glass in the salt solution and 
float the embryo into this. Remove the watch glass carefully 
from the large dish and examine the embryo with a dissecting lens. 
If the vitelline membrane has not yet separated from the blasto- 
derm, it should be removed at this time with fine-pointed forceps 
and needles. Make sure that the embryo lies dorsal side up, as 
it did when the egg was opened. 

4. Slide a cover glass under the embryo, and remove all salt 
solution with a pipette, taking care that the embryo lies in the 
center of the cover glass. Lift the cover glass by one corner so 
that the overhanging edges of the blastoderm fold under, and 
place it in a dry watch glass on a piece of thin absorbent tissue 
paper and add fixing fluid at once. While the embryo is becoming 
attached to the cover glass, remove the yolk, albumen, and pieces 
of shell from the dish of salt solution to a slop jar, reheat the salt 
solution to 40 C., and prepare another embryo. Three embryos 
of each stage are to be prepared. 

5. After five minutes, drop the cover glass, embryo side up, 
into a small stender dish of Bouin's fluid and leave from two to 
four hours. 

6. Rinse in 50 per cent alcohol, wash for two days in 70 per 
cent alcohol to which lithium carbonate has been added or until 
the yellow color is extracted from the embryo, and preserve in 
80 per cent alcohol. 


THE PIG. Embryos of 6 mm. body length and over are easily 
located in the uterine wall. Slit open the uterus and remove the 
embryo with fine-pointed forceps and a horn spoon, taking pains 
not to rupture the membranes. Place at once in Bouin's fluid. 
Embryos of 10 mm. body length should be fixed for four hours. 
Rinsing and preserving are done as for the frog or chick. Larger 
embryos should have the body cavity slit open to admit the fix- 
ing fluid. Fetal pigs of 6 inches or more should be injected 
through the umbilical artery with formalin (20 per cent aqueous 
solution). This solution is also injected into the body cavity and 
cranium, after which the fetus is submerged in the same medium 
for a week and preserved in 6 per cent formalin. 


It is very helpful to have some embryos mounted entire for 
comparison with the serial sections. In making these whole 
mounts, the embryos are stained, cleared, and mounted, i.e., 
transferred to a final medium for preservation and examination 
on the slide beneath a cover glass. 

THE FROG. Frog eggs and embryos may be mounted as opaque 
objects with the natural pigmentation, or they may be cleared 
and stained as transparent mounts. 

Opaque mounts. 

1. Prepare a saturated aqueous solution of thymol. Filter 
the solution, and add gelatin until saturated. Remove the 
supernatant liquid. 

2. Liquefy the gelatin by immersing a small quantity, in a 
test tube, in a dish of hot water. Fill a hollow-ground depression 
slide with gelatin and allow to cool. 

3. With a hot needle, melt a small hole in the gelatin, suffi- 
ciently large to hold the embryo. Place the embryo in the 
desired position and hold it in place until the gelatin has cooled. 

4. Add a drop of gelatin just warm enough to be liquid and 
cover with a cover glass which has been slightly warmed. When 
the gelatin has cooled, any surplus may be removed from the 
edges of the cover glass with a toothpick wrapped in moist cotton. 
In order to prevent the later formation of bubbles, the edges 
of the cover glass should be painted with gold size or Valspar. 

Free-hand sections and dissections are admirably mounted by 


this method, but great care must be exercised to prevent the 
formation of air bubbles through cracks in the gold size. 
Transparent stained mounts. 

1. Bleach the embryo, until white, in hydrogen peroxide. 
About one week is required for this purpose. Embryos that have 
been preserved in 80 per cent alcohol should first be passed 
through 70 and 50 per cent alcohol to water, an hour or more in 
each fluid. Embryos in formalin must be rinsed in water for 
one hour. 

2. Stain in dilute borax carmine four days or more. 

Borax, 4 per cent aqueous solution 100 cc. 

Carmine 1 gr. 

Boil until dissolved and add alcohol, 70 per cent 100 cc. 

To dilute, take 5 cc. of the borax carmine and 95 cc. of 35 per cent 
alcohol and add a crystal of thymol. 

3. If overstained, remove the surplus color with hydrochloric 
acid (1 per cent solution in 70 per cent alcohol) after passing 
through water and 50 per cent alcohol, an hour each. 

4. Run up through 80, 95, and 100 per cent alcohol, an hour 
each, and place in xylene (xylol) until transparent. 

5. Prepare a mounting diagram by drawing an outline of a 
slide on a piece of cardboard and in this laying off an outline of 
the cover glass to be used. Place a clean slide on the diagram, 
and, just inside the right and left margins of the cover-glass out- 
line, attach a thin strip of celluloid, 15/1000 of an inch in thick- 
ness, by means of a drop of acetone. Greater thicknesses may 
be obtained by attaching other strips as necessary. When these 
supports are dry, place a few drops of Canada balsam, dissolved 
in xylene, between the supports, place the embryo in position, 
and lower a clean cover glass gently. Try to avoid the formation 
of air bubbles. If these appear later they may be removed by a 
needle which has been heated or dipped in xylene. A little fresh 
balsam may be run into the cavity. 

THE CHICK. Total mounts may be stained either with the 
borax carmine or with Conklin's modification of Delafield's 
hematoxylin. Delafield's hematoxylin, which gives a blue color 
to the embryo, is made as follows: 


Hematoxylin (16 per cent solution in 100 per cent al- 
cohol) 25 cc. 

Ammonia alum (saturated aqueous solution) 400 cc. 

Hydrogen peroxide, neutralized 25 cc. 

Glycerin 100 cc. 

Alcohol methyl 100 cc. 

Conklin's modification consists of diluting the stain with four 
times the volume of distilled water and adding to each 100 cc. of 
the dilute stain 1 cc. of picrosulphuric acid, prepared by adding 
2 cc. of sulphuric acid to 98 cc. of picric acid (saturated aqueous 
solution) . 

1. Run the embryo from the 80 per cent alcohol down to 
water through changes of 70 and 50 per cent alcohol, an hour 

2. Stain in borax carmine, undiluted, over night, or in hema- 
toxylin from one to three hours. Either stain may be diluted 
still further and the staining period prolonged. In the author's 
laboratory the schedule demands a four-day staining period and 
the borax carmine is diluted 5 X, the hematoxylin 20 X. 

3. Destain, if necessary, in acid alcohol until the desired color 
is obtained. Embryos stained with hematoxylin will turn red 
in the acid alcohol, and the blue color must be restored by wash- 
ing them in running water or, after washing in neutral 70 per 
cent alcohol, placing them in alkaline alcohol (1 per cent ammonia 
in 80 per cent alcohol). 

4. Run up the alcohols, 80, 95, and 100 per cent, half an hour 
each. Pour off half the 100 per cent alcohol and add an equal 
amount of xylene. When the diffusion currents disappear, trans- 
fer to pure xylene and leave until the embryo is transparent. In 
rainy weather, or when 100 per cent alcohol cannot be obtained, 
phenol-xylene (phenol crystals, 25 gr. and xylene 75 cc.) may be 

5. Remove the embryo from the cover glass (if it has not al- 
ready detached itself) and trim the surrounding blastoderm to 
the form of an oblong or circle. Arrange a clean slide on the 
mounting diagram, as described for the frog, attach celluloid 
support, and mount the embryo in Canada balsam with the same 
side uppermost as when the egg was opened. Put the slide away 
where it may lie flat and free from dust until the balsam has 
hardened. This will take at least a week, after which the slide 


may be cautiously cleaned and studied. The process may be 
hastened by drying the slide in the paraffin oven. 

THE PIG. Embryos up to 10 mm. body length may be pre- 
pared as whole mounts by staining in dilute borax carmine, de- 
staining until only a trace of color persists, and mounting in 
Canada balsam. The time spent in each alcohol should be at 
least an hour for the larger embryos. 


In the preparation of serial sections of an embryo, the fixed 
material is (1) embedded in a suitable matrix and (2) sliced into 
extremely thin sections, which are (3) mounted in serial order 
upon slides. The embryo may be stained before or after 

Embedding. There are two principal methods of embedding, 
in paraffin or in celloidin. For especially delicate objects, the 
best results are obtained by a combination of these methods, 
the embryo being first impregnated with celloidin in order to 
avoid the shrinkage (about 10 per cent) caused by paraffin em- 
bedding, and the block of celloidin then immersed in paraffin so 
that ribbons of serial sections may be cut. 

Embedding in paraffin. In preparing the first few embryos 
for sectioning, it is advisable to stain, dehydrate, dealcoholize, 
and clear as if for a total mount. Later, the staining may be 
omitted until after the sections are affixed to the slide. 

1. After clearing in xylene, which should be done in a warm 
place, for example, the low-temperature oven at about 40 C., 
pour off half the xylene and add an equal amount of paraffin chips. 
In the author's laboratory a paraffin of about 55 melting point, 
obtained by mixing commercial paraffin with parawax, is used. 
The parawax, unfortunately, varies in melting point, so that the 
formula is empirical. The embryo may be left in this xylene 
paraffin for two days. 

2. If the mixture has hardened it should again be melted in 
the low-temperature oven. Fill a clean stender dish with melted 
paraffin, transfer the embryo to this, and place in the high-tem- 
perature oven at about 56 C. (or one degree above the melting 
point of the paraffin used) for not more than two hours. The 
xylene paraffin should be thrown in the slop jar. Take care not 


to get any xylene in the high-temperature oven or paraffin used 
for the final embedding. 

3. Smear the interior of a small watch glass with a 10 per cent 
aqueous solution of glycerin (or vaseline), and fill with fresh 
melted paraffin. Transfer the embryo to this, making any neces- 
sary adjustments in position with a heated needle. Place the 
embryo dorsal side up, and note the position of the head. Cool 
the surface of the paraffin by blowing on it gently until it is con- 
gealed. Then plunge it immediately into a dish of cold water or 
waste alcohol and leave it there for five minutes. Mark the block 
for identification. Objects may be left in paraffin indefinitely. 

4. On removing the block of paraffin from its container, 
examine for the following flaws : 

a. Air bubbles, if they are not near the embryo, may be re- 
moved with a hot needle. Otherwise it is better to trim the 
block close to the embryo, put it into melted paraffin, and 

b. Milky streaks are due to the presence of xylene. These 
will crumble during sectioning, so that it is best to re-embed if 
they occur near the embryo. 

c. If the paraffin has " fallen " in the center, it is because the 
surface was cooled too long before the block was immersed in the 
water. If any part of the embryo is exposed, it must be re- 

Sectioning after paraffin embedding. Before sectioning your 
first embryo, be sure you understand the mechanism of the 
microtome (there are many varieties, of which the rotary type is 
best adapted to beginning students), and have practised the 
technique on a block of paraffin. There are three standard planes 
of sectioning corresponding to the axes of the body (Fig. 238). 
Transverse sections are obtained by cutting the cephalic end of 
the body first, with the knife entering the left side. Sagittal 
sections are made by cutting the right side first, with the knife 
entering the ventral surface. Frontal sections are made by 
commencing at the ventral surface, the knife entering the left 
side. It is best to begin with transverse sections. 

1. Attach the paraffin block to the object-carrier of the micro- 
tome in the proper manner to obtain the type of section desired. 
This is done by heating the surface of the carrier until it will just 



melt paraffin, pressing the block against it in the desired orienta- 
tion, and lowering into a dish of cold water. A little melted 
paraffin may be poured around the base of the block and this 
again cooled to secure additional support. 

2. Place the object-carrier in the microtome and, after orient- 
ing the block with respect to the knife, trim it so that the end 
of the block is a perfect rectangle with one of the longer sides 
parallel to the knife edge. If one of the angles is cut off slightly 
there will be a series of indentations in the ribbon which will 
assist in orienting the sections on the slide. 

3. If microtome knives are not available, place a new safety- 
razor blade (Autostrop type) in the holder provided, allowing the 




FIG. 238. . Diagram to show method of orienting embryo with reference to micro- 
tome knife according to type of section desired. 

edge to project between a sixteenth and an eighth of an inch. 
Screw the holder in the knife-carrier so that the edge of the blade 
is tilted inward about 10 from the perpendicular. 

4. Set the regulator for 20 microns (thousandths of a 

5. Run the feed screw as far back as it runs freely; do not 
force it. 

6. Advance the knife-carrier until the edge of the blade just 
clears the block. 

7. Release safety catch and turn the wheel steadily until the 
knife begins to cut the block. Cut slowly, making necessary 
adjustments to the block and knife until you are cutting a per- 
fectly straight ribbon without wrinkles or splits. The principal 
causes of trouble and their remedies are as follows: 


a. The ribbon curls to right or left. This happens because (1) 
the block is thicker on the side away from which the ribbon 
curls, or (2) the knife is duller on the side toward which the ribbon 
curls. Remedy: (1) trim the sides of the block parallel; (2) 
shift the knife to one side. 

b. The sections curl and the ribbon is not continuous. This 
is due to (1) too much tilt of the knife, (2) too hard a grade of 
paraffin, or (3) too cold a room. Remedy: (1) lessen tilt of 
knife; (2) re-embed in softer paraffin; (3) move microtome to 
warmer place, light an electric light or micro-bunsen burner 
near microtome, or cut thinner sections. 

c. The ribbon wrinkles badly. This is caused by (1) too little 
tilt to the knife, (2) too soft a grade of paraffin, (3) too warm a 
room, or (4) a dull or dirty knife. Remedy: (1) increase the 
tilt of the knife; (2) re-embed in harder paraffin; (3) move to a 
cooler room, or cool the knife and block by dropping alcohol on 
them and blowing vigorously, or cut thicker sections; (4) clean 
knife edge with cloth moistened in xylene or shift to a, new place 
on the knife. 

d. The ribbon splits lengthwise. This is due to (1) a nick in 
the knife, (2) a bubble in the paraffin, or (3) dirt on the knife 
edge or side of the block. Remedy: (1) shift to new cutting edge; 
(2) paint surface with thin celloidin ; (3) clean knife edge and block. 

e. The sections refuse to ribbon; they fly apart or cling to 
the knife or the block. This is due to the electrification of the 
sections caused by unfavorable atmospheric conditions. Many 
remedies have been suggested; the best is to ground the micro- 
tome to a water pipe. Usually it is advisable to wait for more 
favorable conditions. 

8. Remove the ribbon in 6 inch lengths with a camel's hair 
brush and arrange these in order, shiny side down, in a cardboard 
box cover. Avoid air currents of all kinds. The ribbons may be 
put away in a dust-free place if the room is not too warm. It is 
better to affix them to slides as soon as possible. 

Affixing paraffin sections to the slide. 1. Prepare a mounting 
diagram by laying off the outline of a slide as before, but enclose 
in this the outline of a long cover glass (25 by 50 mm. approxi- 
mately) and leave space for a label on the right-hand side. 

2. Clean a slide thoroughly by washing with acid alcohol 


followed by distilled water. Place this over the mounting dia- 
gram and brush over the surface above the outline of the cover 
glass with the following dilute solution of egg albumen : 

Egg albumen, beaten and skimmed 50 cc. 

Glycerin 50 cc. 

Filter and add Thymol a crystal 

Dilute 2 drops of this to distilled water 25 cc. 

3. Cut the ribbon into lengths about 2 per cent shorter than 
the length of the cover glass. Using the wet brush from which 
most of the albumen solution has been squeezed, pick up these 
lengths arid arrange them on the albumenized slide so that the 
sections will follow each other like the words on a printed page. 
The shiny side of the ribbon should be next to the slide. Great 
care should be taken to lower the ribbon slowly so as to prevent 
the formation of air bubbles beneath it. 

4. Carefully warm the slides on a warming plate or a piece of 
plate glass, previously heated in the paraffin oven, until the sec- 
tions are expanded and perfectly smooth. If bubbles appear 
beneath the ribbon, prick them with a hot needle while the ribbon 
is still soft and hot. Drain off the surplus water, carefully realign 
the sections, mark the slides with a glass-marking crayon, and set 
them away in the low-temperature oven to dry, at least two days. 
They may be kept indefinitely in this condition if not exposed 
to dust. 

Embedding in celloidin. This method is preferred by some 
technicians as no heat is used in the process and the shrinkage is 
less than that resulting from the paraffin method. However, 
thin sections are not so easy to obtain and the sections must be 
handled individually. 

1. Embryos are dehydrated as for the paraffin method. Leave 
in absolute alcohol one day. 

2. Absolute alcohol and ether, equal parts, one day. 

3. Thin celloidin, three days to one week. 

Alcohol, 100 per cent 100 cc. 

Ether 100 cc. 

Celloidin 5 gr. 

4. Thick celloidin, two days to two weeks. 

Alcohol, 100 per cent 100 cc. 

Ether 100 cc. 

Celloidin 15 gr. 


5. Remove the embryo to a small watch glass and pour thick 
celloidin over it. Cover lightly, or place under a bell jar until 
the celloidin is hard enough to cut with a scalpel. 

6. Dip a block of vulcanized fiber in thick celloidin. Cut 
out a block of celloidin containing the embryo from the watch 
glass and, after moistening the end by which it is to be attached 
in ether alcohol, press it firmly against the prepared fiber block. 

7. Pour a little chloroform into a stender dish, add the block 
and embryo, cover tightly, and allow the celloidin to harden in 
the fumes for thirty minutes. 

8. Fill the stender dish with chloroform and cover. Leave for 
thirty minutes. 

9. Pour off half the chloroform and add an equal amount of 
cedar oil. Leave for one hour. 

10. Transfer to pure cedar oil where it may remain indefinitely. 
Sectioning after celloidin embedding. Celloidin sections are 

usually cut with some form of sliding microtome. Be sure to 
study the mechanism and cut a piece of hardened celloidin before 
proceeding further. 

1. Set the knife with a little more tilt than would be used for 
paraffin, and obliquely to the object so that at least half the 
cutting edge will be drawn through the block. 

2. Orient the block upon the object-holder so that the desired 
type of sections may be obtained. The long side of the block 
should be parallel to the edge of the knife. 

3. Cut sections 20 ^ or more in thickness, using a steady 
drawing cut. Mount sections as they are cut. 

Affixing celloidin sections to the slide. This is best done as 
the sections are cut. 

1. Using the mounting diagram as before, rub on a thin film 
of undiluted albumen solution to cover the areas of the cover 
glass. Rub in well with the ball of the finger. 

2. Arrange the sections in order on this area. When this is 
filled, lay a cigarette paper over the sections and press gently 
with another slide. The slides may be kept in a dust-free 

Double embedding in celloidin and paraffin. This process, 
although tedious, combines the best points of the two methods 
already given. 


1. Embed in celloidin according to the method above, omitting 
step 6. 

2. Trim the celloidin block close to the embryo and wash out 
the cedar oil with xylene, three changes in two hours. 

3. Embed in paraffin as described above, commencing at step 2. 

4. Section according to the method given for paraffin. 

5. Affix to the slide according to the method given for paraffin 

Staining serial sections. When the embryo has been stained 
before sectioning, it is only necessary to remove the paraffin (or 
celloidin), replace with Canada balsam, and cover, if the stain 
proves to be satisfactory. Sometimes, however, it is advisable 
to strengthen or weaken the stain or to add a contrasting dye. 

After staining in bulk. 

1. Paraffin sections on the slide should be put in a Coplin 
staining jar of xylene and left until the paraffin is dissolved, up 
to fifteen minutes. 

2. Transfer to a mixture of xylene arid 100 per cent alcohol, 
equal parts, five minutes. 

3. Transfer to 100 per cent alcohol, five minutes. 

4. Examine slide rapidly under microscope after wiping the 
back of the slide. 

a. If the stain is satisfactory: 

5a. Absolute alcohol and xylene, five minutes. 

6a. Xylene, ten minutes. 

7a. Mount in balsam under cover glass. 

b. If the stain is too intense : 

56. Ninety-five and 85 per cent alcohol, one minute each. 

66. Acid 70 per cent alcohol, until stain is correct. 

76. Sections stained in hematoxylin should have the blue 
color restored in alkaline 85 per cent alcohol. 

86. Eighty-five, 95, and 100 per cent, one minute each. 

96. Absolute alcohol and xylene, five minutes. 
106. Xylene, ten minutes. 
116. Mount in balsam. 

c. If the stain is too light: 

5c. Ninety-five, 85, 70, and 50 per cent alcohol, one minute 


6c. Stain until desired effect is secured. 

7c. Distilled water, five minutes. 

8c. Fifty, 70, 85, 95, 100 per cent alcohol, one minute each. 

9c. Absolute alcohol and xylene, five minutes. 
lOc. Xylene, ten minutes, 
lie. Mount in balsam. 

Celloidin sections on the slide should be exposed to the fumes 
of the alcohol-ether mixture for half a minute, dried for one min- 
ute, and placed in a staining jar of 95 per cent alcohol. All other 
operations may be carried on as above except that phenol-xylene 
should be substituted for 100 per cent alcohol. 

Counterstaining after staining in bulk. In order to differen- 
tiate the parts of the embryo more sharply, it is often desirable 
to add a second stain contrasting with the first. The stains that 
have been employed in the previous exercises are nuclear dyes; 
that is, when extracting by acid alcohol, the color will persist 
in the nucleus after it has been washed out of the cytoplasm. 
The second stains affect the cytoplasm and should contrast in 
color with the nuclear stain employed. After borax carmine, a 
0.5 per cent solution of anilin (Lyons) blue in 95 per cent alcohol 
is employed; after hematoxylin, a similar solution of eosin should 
be used. 

1. Proceed as in the preceding section as far as 66. 

2. Destain in acid alcohol until the color persists only in the 

3. Restore the blue color to hematoxylin-stained sections in 
alkaline 80 per cent alcohol. 

4. Eighty and 95 per cent alcohol, one minute each. 

5. Counterstain lightly, dipping the slide into the solution 
repeatedly until a light color persists in the sections, one-half to 
one minute. 

6. Rinse in 95 per cent alcohol, dehydrate with 100 per cent 
alcohol, followed by xylene-absolute, clear in xylene, and mount. 

Staining with Delafield and eosin on the slide. Follow 
directions given for sections stained in bulk (where stain is too 
light), as far as step 6c, and follow with directions for counter- 
staining as given above. 

Staining with Heidenhain's hematoxylin. This is one of the 
most important embryological stains. 


1. Remove the paraffin from the sections and run down the 
alcohols to distilled water. 

2. Four per cent aqueous solution of iron alum, one hour to 
over night. 

3. Rinse in distilled water and place in 0.5 per cent aqueous 
solution of hematoxylin, same time as in the iron alum. 

4. Rinse in distilled water and return to the iron alum until 
sections are a pale gray. Check from time to time by rinsing in 
distilled water and examining under microscope to see that the 
desired structures are still visible. 

5. When sufficiently destained, wash in running water for 
twenty minutes, or in distilled water, with frequent changes, for 
two hours. 

6. Run up the alcohols, clear, and mount. 

Fuchsin and picro-indigo-carmine. This polychromatic stain 
is especially fine for organogeny. 

1. Remove the paraffin and run down the alcohols to distilled 

2. Stain in basic fuchsin, saturated aqueous solution, twenty 

3. Rinse in distilled water and place in picro-indigo-carmine 
for five minutes. 

Picric acid, saturated aqueous solution 50 cc. 

Indigo-carmine, saturated aqueous solution 50 cc. 

4. Pass rapidly through 70, 95, and absolute alcohol into 
xylene-alcohol. The green dye is extracted most rapidly by the 
70 per cent alcohol, the red by the absolute. Only experience 
will teach the right time allowance for each alcohol. 

5. Clear in xylene and mount. 

OppeFs polychromatic stain. This gives beautiful effects with 
older embryos and larvae. 

1. Fix in Bouin. 

2. Stain in bulk with undiluted borax-carmine, one to two 
days. Destain for the same period. 

3. Embed, preferably by the double method. 

4. Cut sections, 15-20 p. 

5. Run down the alcohols to water. 

6. Stain in picro-indigo-carmine, 1 J minutes. 

7. Stain in picro-fuchsin, one minute. 


Picric acid, saturated aqueous solution 50 cc. 

Acid fuchsin, saturated aqueous solution 50 cc. 

8. Wash in distilled water, changed repeatedly, five minutes. 

9. Ninety-five per cent alcohol, two minutes. 
10. Phenol-xylene, xylene, and mount. 


Not the least important part of technique is the keeping of 
exact records covering every technical operation. For each 
embryo there should be a card, giving the following data: 

1. Kind of embryo and stage of development. 

2. Method of fixation, time and date. 

3. Bulk staining, time and date. 

4. Method of embedding, time and date. 

5. Plane and thickness of sections, and date. 

6. Slide staining, time and date. 

7. Method of mounting, and date. 

8. Name of preparator. 


1. Remove embryo from egg in warm normal salt solution. 

2. Fix for two hours in Bouin's fluid. 

3. Wash in 70 per cent alcohol (plus lithium carbonate), at 
least one change, for two days. 

4. Pass through 50 per cent alcohol and water, one hour each. 

5. Stain in dilute borax-carmine or Delafield's alurn-hema- 
toxylin, four days. 

6. Destain in acid 70 per cent alcohol until desired effect is 

7. Wash in neutral 85 per cent alcohol. (The hematoxylin- 
stained specimen is transferred to alkaline 85 per cent alcohol 
until blue color is restored.) Two days. 

8. Dehydrate and clear: 95 per cent, 100 per cent alcohol, 
absolute alcohol-xylene, xylene, twenty minutes each. 

Mount in Canada balsam 

9. Prepare for embedding by pouring off half the xylene and 
adding an equal amount of paraffin chips. Keep in warm place 
up to four days. 


10. Continue by transferring embryo to melted paraffin and 
place in paraffin oven for an hour and a half. 

1 1 . Embed in fresh paraffin and cool in water. Make blocks. 

12. Cut transverse sections 20 ju in thickness on microtome. 

13. Prepare clean albumenized slide, float sections on this in 
order, warm until sections are expanded, remove surplus water. 
Dry for at least two days. 

14. Remove paraffin with xylene, and 

A. Mount in balsam, or 

B. Run down alcohols to 70 per cent and destain. Run up 

the alcohols, through absolute alcohol and xylene and 
xylene, mount in balsam, or 

C. Run down alcohols to water and restain, dehydrate, clear 

and mount, or 

D. To 95 per cent and counterstain for one minute. Dehy- 

drate, clear, and mount. 


Baker, J. R. 1933. Cytological Technique. 

Ballentyne, F. M. 1928. An Introduction to the Technique of Section Cutting. 

Carleton, H. M. 1926. Histological Technique. 

Gage, S. H. 1925. The Microscope, 14th Ed. 

Guyer, M. F. 1917. Animal Micrology, 2nd Ed. 

Lee, A. B. 1929. The Microtomist's Vade-Mecum, 9th Ed. 

McClung, Ch. 1929. Handbook of Microscopical Technique. 

Oppel, A. 1914. Embryologisches Practikum und Entwicklungslehre. 

Hugh, R. 1934. Induced Ovulation and Artificial Fertilization in the Frog. 

Biol. Bull. 66, 22-29. 
Shumway, W. 1926. Fuchsin and Picro-indigo-carmine, a Polychromatic Stain 

for Vertebrate Organogeny. Stain Technology I, 1. 


During the early stages of development, embryos are too small 
to be studied with the unaided eye. Some observations, to 
be sure, may be made with the dissecting lens, but most em- 
bryological work requires the use of the compound microscope. 
Although the student may be familiar with the use of the micro- 
scope from the elementary course in biology, he should never- 
theless review this subject before proceeding further. In addi- 
tion, he should at this time familiarize himself with the simpler 
methods of measuring objects with the aid of the microscope, 
as embryological drawings require a strict accuracy as to propor- 
tions. A great convenience in embryological work is the camera 
lucida or some other device by means of which accurate outlines 
may be traced. Finally, we must consider the methods by which 
the embryo may be reconstructed in magnified form from serial 
sections, thus returning, in a sense, to the point where the study 
of embryological technique was begun. 


Nomenclature of the microscope. The separate parts of the 
microscope (Fig. 2*39) may be grouped into two systems, the 
mechanical parts, and the optical parts. The principal mechani- 
cal parts are the base, from which arises the pillar, attached to 
which is the arm, which may be inclined at the joint. Attached 
to the arm, just above the joint, is the stage, upon which the 
slide is placed for examination, and beneath this, the movable 
sub-stage equipment, consisting of a condenser-sleeve, and one 
or two iris-diaphragms, by means of which the amount of light 
to be used is regulated. At the base of the arm is the mirror, a 
silvered double mirror, with a plane surface on one side and a 
concave surface on the other. At the upper end of the arm are 
two screws, the coarse and fine adjustments, by means of which 
the barrel of the microscope may be raised or lowered either 








rapidly or very slowly. The barrel is composed of the body- 
tube, connected to the arm by a rack and pinion, in the upper 
end of which is enclosed an inner tube, the draw-tube, on which 
is a graduated scale of millimeters representing the tube length 
exclusive of the revolving nose-piece at the lower end. The 
optical parts of the microscope 
are systems of lenses, the con- 
denser, placed in the condenser- 
sleeve, the objectives, attached 
to the revolving nose-piece, and 
the oculars, one of which is placed 
at the upper end of the draw- 

The condenser. This is a 
system of lenses which increases 
the amount of illumination 
thrown upon the object, and is 
required only with the higher- 
power objectives. 

The objectives. These are 
systems of lenses which produce 
an enlarged and inverted image 
of the object under proper con- 
ditions. Objectives were for- 
merly marked by arbitrary letters FIG. 239. Diagram showing parts of 
or numbers, with the lowest-pow- * he m P und microscope. (From 

i A.- u j-u Gage.) 

er objectives beginning the series. 

To-day they are usually indicated by the equivalent focal length 
(E. F.), that is, the focal length of a simple lens at 250 mm. or 10 
inches, or else by the actual magnification (x) at 160 mm. 
(Leitz microscopes, 170 mm.). In some of the older microscopes 
the tube lengths indicated on the draw-tube were calibrated 
without including the length of the revolving nose-piece, then an 
accessory part. When setting up these instruments the length of 
the nose-piece (Leitz, 18 mm.) must be deducted and the draw- 
tube set at the reduced length (Leitz, 152 mm.). The most 
useful objectives for general embryological purposes are the 
25-mm. or 6 X, which will hereafter be spoken of as the lower- 
power objective; the 16-mm. or 10 X, which will be called the 


medium-power objective; and the 4-mm. or 40 X, known as the 
high-power objective. For the study of the germ cells, an oil- 
immersion objective, of which the front lens must be in contact 
with the cover glass by means of a drop of cedar oil, is necessajy. 
The most generally used immersion objective is that of 1.9 mm. 
E. F. or approximately 95 X . 

Oculars. These are systems of lenses which magnify the 
real image formed by the objective. Like objectives, these 
were, in the past, usually numbered or lettered, beginning with 
that of the lowest power, but now are marked with the E. F. at 
250 mm. or the actual magnification at 160 mm. (Leitz oculars, 
170 mm.). The most useful oculars are the 50-mm. (5 X) or 
low-power ocular, and the 25-mm. (10 X) or high-power ocular. 
When used with the objectives given above, a range of magnifica- 
tion from 30 X to 450 X may be obtained. A method of ob- 
taining the exact magnification will be described in connection 
with the directions for reconstruction given below. 

The use of the microscope. 

1. Place the microscope squarely in front of you with the pillar 
toward you and the stage horizontal. 

2. Place the low-power ocular in the draw-tube, and adjust 
this to a length of 160 mm. (170 mm. for Leitz instruments) as 
indicated on the millimeter scale. Swing the low-power ob- 
jective into position. Place the mirror bar in the median line 
and adjust the mirror to secure an even illumination. Use the 
plane side of the mirror. The concave side is employed only 
when the condenser is not in use. 

3. Place the slide on the stage so that the object to be examined 
is in the center of the stage aperture, and fasten it down with the 
spring clips provided. With the coarse adjustment, lower the 
body-tube until the objective nearly touches the cover glass. 
Then, with the eye at the ocular, slowly raise the body-tube until 
the object comes into plain view. With the fine adjustment, 
raise and lower the body-tube a little at a time until the point 
at which the smallest details show clearly is discovered. This 
is the focal point. 

4. When using the low-power and medium-power objectives, the 
condenser should be lowered until the illumination is evenly dis- 
tributed. With the high-power objective, the condenser should 


be raised almost to the level of the stage. The iris diaphragm 
should be open sufficiently to illuminate about three-quarters of 
the aperture of the objective. In other words, it is more widely 
open for the low-power objective than for the high-power objective. 

5. If a greater magnification is desired, change to the high- 
power ocular, which will double the magnification. If this is not 
sufficient, return to the low-power ocular and swing the medium- 
power objective into position, and so on. On most modern 
instruments, the objectives are par-focal; that is to say, the 
lengths of the objectives are such that when another objective 
is swung into place the object will still be visible. If, however, 
the object is not in focus, it is best to lower the body-tube until 
the new objective almost touches the cover glass, and focus up 
until the object comes into view. If the oil-immersion objective 
is to be used, lower the condenser and place a drop of oil on its 
upper surface; then raise it until it touches the bottom of the 
slide. Place another drop immediately over the object on the 
cover glass and lower the body-tube with great care until the 
front lens of the objective touches the oil. Focus by means of 
the fine adjustment only. 

6. All optical parts of the microscope must be cleaned with 
lens-paper. After the oil-immersion objective has been used, 
the front lens, condenser, and slide should be wiped with a bit 
of lens-paper dipped in xylene and then dried with a fresh piece. 
Never separate any of the optical parts. The microscope should 
be lifted by the pillar unless a special grip is provided to the arm. 
The microscope should be kept in the case when not in use. One 
of the oculars should be left in the draw-tube at all times to 
prevent dust getting on the upper lenses of the objectives. Be- 
ginners should try to avoid the error of closing the eye that is not 
in use. Practice will enable the microscopist to work with both 
eyes open and even to alternate the right and left eye at the 

Micrometry. The unit of measurement in microscopy is the 
micron (M). It is the one-thousandth part of a millimeter. 
Measurement of microscopic objects is performed with the aid of 
micrometers, of which there are two types, the stage micrometer 
and the ocular micrometer. The former is a glass slide, in the 
center of which, under a cover glass, is a line, usually 2 mm. long, 


divided into 200 equal parts, each of which, therefore, is equivalent 
to 10 fjL. The ocular micrometer is a glass disc, placed in an 
ocular at the level of the ocular diaphragm, on which is engraved 
a scale, with arbitrary subdivisions. Some oculars are furnished 
with a draw-tube so that the upper lens of the system may be 
focused more sharply upon the scale. The value of the divisions 
indicated on the scale varies according to the amount of magnifi- 
cation of the real image, and so must be obtained for each ob- 
jective independently, according to the following method: 

1. Arrange the microscope as before, taking particular care to 
secure the proper tube-length. 

2. Focus the eye-lens on the ocular micrometer scale by means 
of the ocular draw-tube. Focus the objective on the stage mi- 

3. Make the lines of the stage micrometer parallel with those 
of the ocular micrometer, and determine the value of the divisions 
of the ocular micrometer in terms of those of the stage microm- 
eter. Thus, if it requires 10 spaces of the ocular micrometer, 
and the latter is equal to 0.1 mm., then the value of a single 
space of the ocular micrometer for that particular objective and 
at that particular tube-length is 0.01 mm. or 10 M. Determine 
the value of the ocular micrometer for each objective in the same 


Free-hand drawings of microscopic objects can only approxi- 
mate an accurate representation. However, great pains should 
be taken to secure at least accurate proportions, neat and clean- 
cut lines, and complete labels. Accurate outlines can be secured 
by the aid of the camera lucida, various types of projection 
apparatus, or microphotography. 

Equipment. The student will need a hard lead pencil (4H), 
a medium pencil (HB), and blue, red, and yellow colored pencils, 
an eraser, and bond paper to fit the note-book cover used in 
earlier courses. 

Free-hand drawing. 

1. Lay off the space to be occupied by the drawing, by placing 
four dots at the corners. Rule in two lines, intersecting at the 
center of this space. These will represent the dorso-ventral 


and the dextro-sinistral axes, if the drawing is to be of a transverse 


2. Measure the corresponding axes of the sections by means 
of the ocular micrometer, multiply by the desired magnification 
of the drawing, and lay off these magnified measurements on the 
cross lines already drawn. The following magnifications are 
recommended: for the twenty-four hour chick, 100 X; for the 
thirty-three hour chick, 75 X; for the forty-eight hour chick, 
50 X ; for the seventy-two hour chick, 30 X ; for the 10 mm. 
pig, 20 X. 

3. Draw in a careful outline of the section and of the internal 
structures, paying particular attention to the proportions, which 
should be measured with the ocular micrometer and laid off on 
the axes at the proper magnification. 

4. On one side of the dorso-ventral axis, all structures should 
be colored with the crayons in accordance with the following 
scheme: ectoderm, blue; mesoderm, red; and endoderm, yellow. 

5. Label all structures represented in the section, using broken 
lines at right angles to the long axis of the paper to connect the 
label with the structure indicated. 

6. Identify the drawing fully, by means of a serial number, 
the species, and stage of development, the number given to the 
series, slide, and section, the type of sections, and the amount of 
magnification. Example: No. 23, Chick, 48 hours, Series 1102, 
Slide 2, section 28, transverse section 50 X. If a drawing has 
already been made of the total embryo or a total mount, indicate 
on this, by means of a heavy ruled line, the position of the section 
just drawn, and number this line with the serial number of the 

Abbe camera lucida. This is an attachment which reflects 
the light from the drawing board, by means of a mirror, to a 
silvered prism, whence the light is reflected to the eye, super- 
imposed on the image of the object which is transmitted through 
a small hole in the silvered surface of the prism directly above the 
ocular of the microscope (Fig. 240). 

1. Attach the camera to the draw-tube of the microscope in 
such a way that the mirror projects to the right, and the opening 
in the prism lies above the center of the ocular. 

2. Extend the mirror a,rm to its greatest length and set the 



mirror at an angle of 45. The mirror arm must be parallel to 

the drawing board. 
3. Try various combinations of objectives and oculars until an 

image of the desired magnification appears on the paper. Magni- 
fications intermediate to those ob- 
tainable in this way may be secured 
by varying the tube-length or by 
raising or lowering the drawing 
board. If the stage of the micro- 
scope interferes with the drawing, the 
mirror should be set at an angle of 
40 or 35 and the drawing board 
tilted toward the microscope at an 
angle of 10 or 20, respectively, by 
means of wooden images. If the 
image is stronger than the reflection 
of the pencil point, a smoked glass 
may be placed beneath the prism, or 
the aperture of the iris diaphragm 
may be reduced. If the reflection of 
the pencil is stronger than the 
image, smoked glass may be placed 
at the side of the prism or the amount 

FIG. 240. Diagram showing prin- of light falling on the paper reduced 

ciple of the Abb6 camera lucida. by means of a screen. 

Path of image seen in microscope 4> Draw ^ ^ Qutlines of the gec . 

shown in broken lines, that on 

drawing paper shown in unbroken tions and the larger internal struc- 

Hnes. (From Gage.) tures. The details may be added 


5. Remove the slide and substitute a stage micrometer. Trace 
in part of the scale by means of which both the magnification 
of the drawing and the absolute size of the object may be computed 

Projection apparatus. Where many drawings are to be made, 
as in the case of reconstructions, some form of apparatus by 
means of which the image of the section may be projected 
directly upon the paper is very helpful. There are many types of 
projection apparatus, directions for the use of which may be 
obtained with the instruments. 


Microphotography. The photography of minute objects with 
the aid of the microscope is of great assistance in embryology. 
However, the methods are so difficult, the apparatus so complex, 
expensive, and delicate, and the process requires so much tech- 
nical knowledge and skill, that microphotography has been con- 
sidered a field too advanced for the beginning student, although 
a method described by Headland seems to overcome these 
difficulties to a large extent. In recent years the motion-picture 
camera has been adapted for use with the microscope, and 
excellent results have already been obtained. 


After an embryo has been sectioned, it is sometimes necessary 
to reconstruct some part of it from the sections. There are two 
important methods: graphic reconstruction, in which a geo- 
metric projection of a sagittal section, for example, might be 
made from transverse sections; and plastic reconstruction, in 
which magnified replicas of each section are made of wax and 
piled together so as to make an enlarged model of the object to 
be studied. A complete series of sections of uniform thickness 
and accurate orientation is required for either type of reconstruc- 
tion, and an outline drawing of the embryo before sectioning is 
of great assistance. 

The graphic method (of His). This method can best be 
described by giving practical directions for a particular problem, 
e.g., to prepare a geometrical sagittal projection 20 X of the 
neural tube of a 10 mm. pig embryo from a series of transverse 
sections 20 M in thickness. 

1. From the lateral view of the embryo drawn before sectioning, 
make an outline drawing 20 X . 

2. Draw a median line corresponding to the cephalo-caudal 
axis, the length of which, in this case, should be 200 mm. 

3. Count the number of sections in the series, in this case, 500. 

4. Locate the position of each transverse section which you 
have drawn on the median line of the outline. Thus if the most 
anterior section drawn was the fiftieth of the series of 500 sec- 
tions, it would be located at a point 1/10 of the total length of 
the axis (200 M)> or 20 mm. from the cephalic end. 

5. Theoretically, each section is at right angles to the median 


line, but this angle may be greater or less as a result of variations 
in technique. Study each drawing of a cross section in connec- 
tion with the "drawing of the total embryo and determine the 
angle made by that section with the cephalo-caudal axis of the 
embryo. Draw in ; at each point located on the median line, a 
cross line at the proper angle so determined. These lines repre- 
sent the dorso-ventral axes of the transverse sections. Their 
lengths should correspond with those of the dorso-ventral axes 
of the drawings of the transverse sections previously made at 
the same magnification, 20 X. 

6. Plot in on each section-plane line (dorso-ventral axis) the 
dorsal and ventral boundaries of the neural tube as determined 
from measurements of the drawings already made. Interpolate 
by direct measurement and magnification of these points on 
intervening sections. 

7. Sketch in the contours of the neural tube by connecting up 
the points which have just been plotted. Compare the drawing 
with a sagittal section of an embryo in the same stage of develop- 

Plastic reconstruction. This method also will be indicated 
by practical directions for the reconstruction of a particular 
organ, in this case, a model 50 X of the heart of a 10 mm. pig, 
from a series of transverse section, 20 ^ in thickness. 

1. Prepare a number of wax plates of the proper thickness. 
In this case, if every section is to be reconstructed, the thickness 
of the plates must be 50 X 20 /z, or 1 mm. Nearly as good results 
can be obtained by reconstructing every second section and 
making the plates twice as thick. The wax is prepared according 
to the following formula: 

Beeswax 6 parts 

Paraffin, 56 C. m.p 4 parts 

Rosin, white lump 2 parts 

Mix and melt. 

Pour 130 grams of this wax into a pan with an inside measure- 
ment of 500 X 280 mm., into which boiling water has been poured 
to a depth of 15 mm. This amount of wax will make a plate 
1 mm. in thickness. Bubbles in the wax may be removed by 
playing the flame of a bunsen burner over the surface as it is cool- 
ing. As the surface hardens, cut the edges free from the sides of 


the pan. When the wax has set out is still malleable, roll up 
the plate and remove it to a soapstone slab, where it is unrolled 
and allowed to cool. 

2. With the help of a camera lucida or projection apparatus, 
prepare outlines 50 X of the heart in all the sections in which it 
is found. Number the drawings consecutively and note the serial 
number of the sections drawn, so that it will be possible to 
check the drawings later if necessary. Note also whether the 
right and left sides of the drawing actually correspond with the 
right and left sides of the embryo or whether this condition is 
reversed. This is very important, as a mistake at this point 
would render the reconstruction valueless. 

3. Transfer the drawings to the wax plates by means of carbon 
paper. Place the wax plates on a sheet of glass, and cut out the 
parts to be preserved with a sharp scalpel, leaving bridges of wax 
to connect the parts which would otherwise be separated. These 
bridges are best made in the form of arches bending towards the 
outside of the section. 

. 4. Pile the sections in order, taking care to avoid the reversal 
of right and left sides, and to get an accurate fit. It is best to 
group the sections in piles of ten. A steady pressure of the hand 
will be sufficient to cause the sections to adhere to each other. 
The bridges may be cut away and stout pieces of wire sub- 
stituted. Heat the wire at each end and press into position. 
After the wire is set, the wax bridges are cut away and the edges 
of the piece smoothed with a heated scalpel or aluminum modeling 

5. When all the sections have been combined in groups of ten, 
these groups should be united and the completed model smoothed 
in the same way. Such models may be painted or dissected, 
and mounted on wooden supports as desired. They are quite 
permanent if not exposed to high temperatures. Plaster of 
Paris molds and casts may be made from them in the customary 


Belling, J. 1930. The Use of the Microscope. 
Gage, S, H. 1932. The Microscope, 15th Ed. 
Guyer, M. F. 1917. Animal Micrology, 2nd Ed; 


Headland, C. I. 1924. A Simple and Rapid Photomicrograph for Embryological 

Sections. Anatomical Record, XVII, 2. 
Lee, A. B. 1929. The Microtomist's Vade-Mecum, 9th Ed. 
Mueller, J. F. 1935. A Manual of Drawing for Science Students. 
Norman, J. R. 1923. Methods and Technique of Reconstruction. Journal of the 

Royal Microscopical Society. 


References to figures have an asterisk (*) prefixed to the page number: Com- 
pound terms are indexed under the first word, e.g., dorsal aorta not aorta, dorsal. 
.Synonyms are distinguished by italics and referred to the terms employed by the 
author. See also the Glossary. 

Abbe" camera lucida, 355, *356 
abdominal cavity, *198 
abducens nerve, *256, *257, 266 
aberration, see chromosomal aberration 
accessory nerve, *256, *257, 258 
acoustic, see also auditory, otic 
-4- ganglion, 257, *319 

nerve, *256, *257, *291 
acrosome, 34, *35, 54 
Adamstone, 176 

adnexa, 126, *139, *141, *147, *148; 
see also extra-embryonic appendages 
adrenal gland, *206, *213, 214 
adrenalin, see epinephrin 
after-birth, 146, 151 
agglutination, 53 
air chamber, *40 
albumen, 38, *39, *40, *100 

sac, *139, 142, 143 
alimentary canal, see digestive tube 
aUantoic artery, 142, *219, *227 

blood vessels, 146 

stalk, H5, 192 

vein, 142, *222, *227, *313 
allantois, 42, 140, 142, 146, 147, 187 

in chick, 21, 142, *143, *190, *311, 

in mammals, 143, 145 ff 

in man, 25, *141, *147 

in pig, *145, *325 
allelomorph, 73 ff 

Ambystoma, 153, 175, *176, 253, *254, 



amniocar&Jac vesicle, 138, *295 
amnion, 42, 137, *138 

~~ ^ % 140 ' * 141 

in (|ic%21, 137, *139, *143, *304 ff 

amnion, in dog, *146 

in man, 25, 140, *141, *148 

in marsupials, *145 

in pig, *145 

in rat, *137 
amniotic fluid, 137 

folds, *141, 142, *303 

raphe, 137, *308, *314 
amphioxus, life history, 12 ff, *14 
amplexus, 52 

ampulla, 267, *268 
anaphase, *61, 62, *66 
anestrum, 210 
anilin blue, 346 
animal pole, see pole 
anterior cardinal vein, 216, *217, 222, 

in chick, *227, *305, *312 

in pig, *320, 321, 327 

caval vein, see precaval 

chamber (of eye), *266 
antipole, see pole 

anus, 193 

aortic arch, 216, *217, *219, *220 

in frog, *284 

in chick, *305, *311, *312 

in pig, *318, *320, *327 

roots, 219 

appendages (of body), 129 
appendicular muscles, *238, *240 

skeleton, *235, 236 
appendix testis, *209, 213 
aqueduct, see mesocoel 
aqueous humor (of eye), 266 
archencephalon, 250, *251 
archenteron, see gastrocoel 
arcualia, *230 

area opaca, 105, 112, 122, *123 

pellucida, 105, 122, *123, *295 




area vasculosa, 122, *123, 135, 136, *143, 

of allantois, 142 

vitellina, *123, 126 
Aristotle, 3 

arteries, *217, *219, *225, *227, *228 

articular bone, *234, 269 

artificial parthenogenesis, 7, 56, 154, 


Ascaris, 43 

assortment (of genes), 75, *77 
aster, *61, 62 
Astylosternus, 183 
asymmetry, 157 
atriopore, 132, 157 
atrium, 218 

in frog, *284, *291 

in chick, *303, *307, *309, *311, *312 

in pig, *318, *320, *322, *323, *327 
attachment-region (of chromosome), *61, 


attraction (of sperm), 52 
auditory, see also acoustic, otic 

ossicles, *268, 269 

tube, 184, *263, *268, 269 
auricle, see atrium 
autonomic ganglia, 254, *255 

nerves, 254, *255 
autoplastic, see homoplastic 
autosome, 69, 87 
auxocyte, see 'cyte 

axial filament, *35 

mesoderm, *120, *297 

muscles, 239 

skeleton, 230 
axolotl, 175, *176 
axon, 248 

azygous vein, *223, 228 


back-cross, 78, *79, *80 

Balfour, 90, 98 

basalia, 235 

basilar artery, *219, 221, *319, *327 

chamber, 269 

plate, 232 
beak, 189, 247 
Bell, 258 
Bellamy, 38, 157 

Bijtel, 132 

bilaterality, 157 

biogenetic law, see recapitulation theory 

bisexual reproduction, 31 

bladder, muscles, 241 

blastocoel, *101, ff 

jelly, 108 

blastocyst, *101, 102, *106 
blastodisc, 98 
blastomere, 89 

blastopore, 106, *109, 113, 115, *117, 

blastula, 89, *101, *102, *104 

in amphioxus, 13, *102 

in chick, 20, *105 

in frog, 16, *103, *104 

in mammals, *106 

in man, 23 

in rabbit, *106 
blood corpuscles, *215 

forming centers, 215 

island, *214, 215, *297, *305 

plasma, 214 

vessels, 215, 216, *2}7 ff 
body form, 126, *127 

in frog, *128, 129, *130 

in chick, 22, 132, *133 

in man, 25, *134 

stalk, *141, 144, 147 
Bombinator, *169 
bone, 34, 229 

marrow, 215 
borax carmine, 337 
Bourn's fluid, 334 
Boyden, 317 
Brachet, 6, 260 
brachial plexus, 250 

brain, 250, *251, *256, *295, *297 

stem, 252 
branchial, see also visceral 

arches, 131, 233 

artery, 226 

branchiomeric metamerism, 258 
Branchiostoma, see amphioxus 
Born, 156 

Bowman's capsule, *202 
Bridges, 88 

bronchi, 184, 190, *192, *312 
bulbus arteriosus, 218, *303, *307, *312, 



calcium-free seawater, 158, 174 

Carothers, 62 

carotid arteries, *217, *219, 222, *319 

gland, 188 
carpalia, *235, 236 
cartilage, 34, 229 

bones, 229, *234 

caudal artery, *217, *219, *326 

vein, 223 
caul, 151 
cecum, 191 
cell, 31, *32 

division, 60, *61 

lineage, 92, *93, 158 

vacuoles, *32 

wall, *32, 34 
celloidin, 343 
cellular embryology, 5 
centrifuge, 157, 171 
centriole, see centrosome 
centrosome, *32, 33, 35, 48, 54, 62 
cephalic flexure, 132, 134, 138 
cerebellum, 252 
cerebrospinal ganglia, 254 
cerebrum, 250 

cervical flexure, 129, 134, 189 

sinus, 317 
chalazae, *39, *40 
chemical embryology, 7 
chemicals affecting development, 173 
chemo-differentiation, 163, 168 
chemotaxis, 52 

chick, life history, 19, *22 

15 hours, *121 

20 hours, *121 

24 hours, 295 ff 

25 hours, *133 

33 hours, 298 ff 

38 hours, *133 

48 hours, 303 ff 

68 hours, *133 

72 hours, 309 ff 
Child, 37, 157 
chondrification, 229 
chondriosomes, see mitochondria 
chondroblasts, 229 
chondrocranium, *232 

chorda dorsalis, see notochord 

chorda-mesoderm, 18, 89, 106, 111, 114, 

115; see also middle germ layer 
chorda-neural crescent, 116 

plate, *109 
chorio-allantois, *139, 143 
chorion of amniota, 137, *138 

of anamniota, *38 

of chick, 21, *139, *304 jf, *312jf 

of man, H41, *148, 149 
choroid fissure, 264, *309 

layer (of eye), 264, *266 

plexus (of brain), 252 
chromaffin cells, 256 
chromatids, 64, *66 
chromatin, *32 

diminution, 43, *45 
chromatophores, 246, *293 
chromidia, 33 
chromomeres, 32 
chromoriemata, 60, *61, *66 
chromosomal aberrations, *85, 86 
chromosome maps, 81, *82 
chromosomes, 60, *61, *66, *68, 158 

of amphioxus, 71 

of chick, *72 

of frog, 71, *72 

of man, *72 
clavicle, 236 
claw, *245, 247 
cleavage, 91 ff, *92, 158JF 

of amphioxus, 13, 95, *96, 158 

of bat, *100 

of chick, 20, 98, *99 

of frog, 16, *97, *98, 159 

of mammals, 99, *100 

of man, 23 

of monkey, *100 

of newt, *160 

of Triton, 161 
cleidoic egg, 42, 142 
clitoris, 211 
cloaca, 187 

of chick, *190, 191, *311 

of frog, *276, *277, *281, *288 

of man, 192 

of pig, *192, *318 
cloacal plate, 187 
closing plate, 183 
cochlea, 267, *268 
cochlear nerve, 267 



coeliac artery, *219 
coelom, *120, 121, 128, 195 

of chick, 198, *302, *307, *308, *313, 

of frog, *198 

of man, 199 

of pig, *324, *326 
Coghill, 253 

collecting tubules, *202, *203 
collection of embryos, 331 
columella (of ear), 233, 269, *270 
common bile duct, 186 

cardinal vein, 216, *217, *223 

of chick, *307, *312 

of pig, *322 

communicating ramus, *254, 254 
comparative embryology, 4 
concrescence, *108, 109, 111, 121, *127 
conjunctiva (of eye), 265 

Conklin, 37, 56, 94, 97, 102, 108, 116, 

157, 159, 163, 338 
connective tissue, 229 
coprodeum (of chick), 191 
coracoid bone, *235, 236 
corium, see dermis 
cornea (of eye), 265, *266 
corneum (of skin), 245 
corona radiata, *41, 207 
coronary sinus, *223 
corpora bigemina, 252 

quadrigemina, 252, 261 
corpus luteum, 207 
cortin, 213 

cranial, see also cerebral 

flexure, 250 

muscles, *239, *240, 260 

nerves, 233, *240, *256, *257, *319, 

skeleton, see skull 

criss-cross inheritance, see sex-linked in- 

crop (of chick), 190 

crossing-over (between chromosomes), 
78, *80, *81 

crypts (of uterine wall), 146 

cumulus oophorus, *207 

cutaneous artery, 226 

cutis, see dermis 

Cynthia, see Styela 

'cytes, *47, see also oocyte, spermatocyte 

cytokinesis, 60, *61, 62 
cytology, 5 
cytoplasm, 32, 156 
cytosome, *32, 33 


Dareste, 172 

de Beer, 8 

dedduae, *148 

deciduate placenta, 146 ff 

deferent duct, 208, *209 

deficiency (of chromosomes), 85 

Delafield's hematoxyliri, 337 

delamination, *107 

deletion (of (Chromosomes), 85 

demibranch, 132, 184; see also internal 


deridrites, 248 
Dentalium, 159, 163 
dental papilla, *182 
dentary bone, 235 
dependent differentiation, 165 
dermal muscles, 239 

skeleton, 230, *234, *235 
dermatome, 196, *238, *314 
dermis, *244 

dermocranium, *234, *235 
descending aorta, see dorsal aorta 
determinate cleavage, *159 
deutencephalon, 250, *251; see also epi- 

chordal brain 

deutoplasm, 34; see also yolk 
diacoel, 252 

diaphragm, *198, 199, 239 
dictyosomes, see Golgi bodies 
Diddphys, 144 
diencephalon, 250, *251 

of chick, *304, *305, *309, *311, 

of frog, *290, *293 

of pig, *316, *318, *320 
diestrum, 210 
digestive tube, 127, 182 ff 

diploid number (of chromosomes), 67 
dipnoi, 263 
discoblastula, *101 

disjunction (division), 67, see also reduc- 

displacements in gastrulation, *119 
dominance, 73, 89 



dorsal aorta, 216, *217, *219, *220 

of chick, *302, *304 ff, *311 ff 

of frog, *289 

of pig, 318, *32lff 

lip, 107, *110, *112, *116 jf, 119 ff, 
166, 169 

double embedding, 344 

embryos, 162, *163 
Drosophila, 72, *74 ff 
ductus arteriosus, *220 

Botalli, see ductus arteriosus 

choledochus, see common bile duct 

Cuvieri, see common cardinal vein 

venosus, *222, *318, *323, *327 
duodenum, 186, *313, *318 
duplicate monsters, 163, 171 
duplication (of chromosomes), 85 
dura mater, 250, 264 

Duval, 122, 294 
dyads, *65, *66, *67 


ear, 250, 261, 267, *268 

of chick, 270 

of frog, 269, *270, *288, *293 

of man, *268, 271 

of pig, *316, *318 
ectoblast, see ectoderm 
ectoderm, 91, 106 ff 
ectodermal derivatives, 116, 244 
of chick, 247, 261, 270, 298, 301, 

308, 315 

of frog, 246, 260, 269, 277, 282, 292 

of man, 247, 261, 271 

of pig, 326 

ecto-mesoderm, 115 

Edwards, 162 

effectors, 248 

efferent ductules (of testis), 206 

egg, *36jF,41jf 

albumen, 38 
solution, 343 

capsules, 38 

envelopes, 38 

jelly, 38, *57 

of amphioxus, *39 

of cat, *36 

of chick, *39, *40 

of frog, *39 

of man, *39, *41 

egg of Myxine, *38 

shell, 38 

tooth, 189, 247 
embryo as a whole, 9 

period of, 134 
embryologists, 3 ff 
embryology, 3 ff 
embryonic disc, *106 

form, *126 

knob, *106, *113 
enamel organ, *182 
endocardium 218, *302 
endoderm, 91, 106 ff 
endodermal derivatives, 116, 181 
of chick, 188, *190, 296, 300, 306, 


of frog, 187, *189, 276, 281, 287 

of man, 191 

of pig, 317 

endolymphatic duct, 267, *268 

endo-mesoderm, 115 

enteric tube, see digestive tube 

enterocoel, 117, *118, 128, 195 

entoblast, see endoderm 

entoderm, see endoderm 

entypy, *137 

enucleate eggs, 154 

environment in embryology, 171 

eosin, 346 

eparterial bronchus, *192, *322 

ependyma, *248, *253 

epiblast, see ectoderm 

epiboly, *107, 109, 112, 120 

epibranchial placodes, 270 

epicardium, 218 

epichordal brain, 232, 250; see also 

epidermis, 120, *244, *296 ff 
epididymis, 208, *209, 212 
epigastric artery, *219 
epigenesis, 4 
epi-myocardium, *302 
epinephrin, 8, 213 

epiphysis, 250, *276, *283, *309, *311 
epithalamus, 251 
epithelial bodies, *182, *183 
eponychium, 247 
epoophoron, 208, *209 
equation division (of chromosomes), 67 
erythrocytes, *215 



esophagus, *182, 184 

of chick, *190, *312 

of frog, *189, *292 

of man, 191 

of pig, *192, *316jf 
estrous cycle, 209 
estrus, 210 

ethmoid bones, *234 

plate, 232 

Eustachian tube, see auditory tube 

evagination, 12 

excretory organs, see nephric organs 

exocoel, 136, 142, 195, *139, *297, *301 ff 

exo-embryo, 174, *175 

exogastrula, 174, *175 

experimental embryology, 6, 153 ff 

explantation, 165, *166 

external auditory meatus, *268, 269 

carotid artery, *217, *219, *220 
of chick, 132, H33, 295, 298, 303, 


of frog, 129, *130, 275, 278, 287 

of man, *134 

of pig, 317 

genitalia, 210 

gills, 188, *281, *284, *286; see also 

extra-embryonic coclom, see exocoel 

structures, 126, *135 ff; see also 

extra-uterine development, 26 
eye, 250, 252, 261, 264, *265 

of chick, 270 

of frog, 269, *288, *290, *293 

of man, *266, 271 

of pig, *316 

eyeball envelopes, 264, *266 

muscles, 239, 256, 266 

Fi 9 F 2 , generations, 73 

facial nerve, *256, *257, 267, 269 

falciform ligament, *197, 199 

false amnioHy see chorion (of amniotes) 

feathers, *245, 247 

female hormones, 207 

femur, *235, 236 

fenestra ovale, 267 

rotunda, 267 

fertilized egg, *55, *56, *57, *58, 153; 

see also zygote 
fertilization, 50, 52, *53 

cone, *53 

membrane, *39, 53 

of amphioxus egg, 13, *56 

of frog's egg, 16, *57 

of guinea pig egg, *58 

of hen's egg, 19, *58 

of human egg, 23, 58 
fertilizin, 53 

fetus, 26, *150 

fibula, *235, 236 

fixing fluid, 333 

flexure, 132, 261 

follicle (of ovary), 38, *50, *207 

foods affecting development, 175 

fore-brain, see prosencephalon 

fore-gut, 122, 127, 136, *181 

of (-hick, *297, *299, *300, *302, *307 

of frog, *276 ff 
fraternal twins, 163 

free appendages, *235, 236 
free-martin, 164 
frog, 3 mm., 275 ff 

6 mm., 278 ff 

11 mm., 287 ff 

life history, 16, *17 
frontal bone, *234, *235 
fuchsin, 347 


gall bladder, 186 
-- - of chick, *190 

of frog, *292 

of pig, *192, *324 
gametes, 31. 

gamctogenesis, 43, 44, 47 
ganglia, 248, *253 

of chick, *312 

of pig, *319 
ganglion of Froriep, 258 
Gartner's canal, 208, *209 
gastral mesoderm, 115 
gastrocoel, 106, *110, 111, 114, 136 
gastro-hepatic omentum, 199 
gastrula, 89, *107 

of amphioxus, 13, 108, *109 

of bat, *113 

of chick, 20, 111, *112 



gastrula of frog, 16, *109, *110 

of man, 23, 113, *114 

of pigeon, 111, *113 

of urodeles, 119 
gastrulation, see gastrula 
genes, 6, 60, 72, 154, 158 
genetics, 5 

genie balance, 88 
geniculate ganglion, 257 
genital artery, 221 

ducts, 187, 208 

organs, 205 ff 

ridge, 205, *206, 212 

tubercle, *211 
genotype, 75 
germ, 3, 31 

cells, 31 ff, 43 

layers, 106 Jf, 115,165 

of amphioxus, 15, 108 ff, 116 ff 

of chick, 21, 111 ff 9 121 ff 

of frog, 18, 109 ff, 118 ff 

of man, 25, 113 ff, 122 ff 

wall, 121, *123 
germinal disc, 40 
germinativum (of skin), *244 
girdles (of skeleton), *235, *240 
gizzard (of chick), 190 

gland (of skin), *246 

glomerulus, 200, *201, *323, *324, *326 

glossopharyngeal nerve, *256, 258, 267, 


glottis, 184 

Golgi bodies, *32, 33, 48, *63 
gonads, 46, 205, *206 
gone, 47 
'gonia, 46, 206; see also oogonia, sper- 


gonoducts, see germinal ducts 
gonomery, *156 

Graaffian follicle, see vesicular follicle 
gradient, 37, 157, 158 
gravity, 171 
gray crescent, 55, *57, 58, *98, 103, 109, 

157, 159, 163 
Gudernatsch, 175 
gynandromorph, *88 


Haeckel, 6 
hair, *245 

haplpid larva, *155 

number (of chromosomes), 67 
Harrison, 165 

Harvey, 4 

hatching of amphioxus, 15 

of chick, 23 

of frog, 18 
head, 129 

fold, 122, 132, 134, *295 

-of amnion, 138, *299, *300 

Headland, 357 

heart, 216, *217, *218 

muscle, 238, 241 

of chick, 266, *299, *300, *304 

of frog, 224, *281, *283, *288 

of man, 227 

of pig, *316, *318 
heat, 172 

ITeidenhain's hcmatoxylin, 346 

hemiazygous vein, 229 

hemipenis, 211 

hemiplacenta, 144, *145 

hemoblast, *215 

Hensen's node (or knot), see primitive 

hepatic-portal veins, *222, *324, *325 

veins, *222, 224 
Herbst, 158, 173 
hermaphrodite, 31 
Hertwig, 90, 154, 156, 172 
hcteroplastic transplantation, 165, *166 
heterotypic division, see meiosis 
heterozygous, 73 

hind-brain, see rhombencephalon 
hind-gut, 127, 136, 142, *181 

of chick, *283, *314 

of frog, *276, *277, *280, *281 
His, 6, 357 

Hisaw, 207 

holoblastic cleavage, *90 

Holtfreter, 174, 183 

homolecithal, see isolecithal 

homoplastic transplantation, 165, *166 

homozygous, 73 

hoof, *245 

hormones in development, 176 

horn, 245 

humerus, *235, 236 

Huxley, 163 

and de Beer, 157 



hyaloplasm, 33 
hybrid vigor, 156 
hymen, 213 
hyoid arch, 131, 233, 269 

cartilage, 233 
hyomandibular bone, 269 

cartilage, 233 

pouch, 131 
hypoblast, see endoderm 
hypochord, 119, 276, *279 
hypoglossal nerve, *256, 258 
hypophysis, *181, 252 

of chick, *305 

of frog, 187, *277, *278 

of pig, *192, *320 
hypothalamus, 251 

identical twins, 163 

idiosome, *47, 48, 50 

ileum, *235, 236 

iliac artery, *219 

implantation, 25, *114, 140, 210 

incubation, 21, 332 

incus, 234, *268, 269 

indeciduate placenta, 144 

indeterminate cleavage, *159 

infundibulum (of brain), *181, 252, *283 

*289, *311, *320 

inheritance of acquired characters, 46 
inner ear, 267, *268 

zone (of area opaca, q.v.) 
insulin, 187 
integument, *24A ff 

intermediate mesoderm, *120, *297, *302; 

see also nephrotome 
internal carotid artery, *217, *219, *319, 


gills, 183, 188, *288, *291; see also 

interplantation, 165, *166 
interrenal gland, *213, 214 

vein, 223 

intersegmental artery, *219, *220 
intersexes, *87 

interstitial cells (of testis), 206 
intestinal portal, 122, *295, *300 
intestine, 186 

of chick, *190 

intestine of frog, *288, *289, *292, *293 

of pig, *324, *325 
invagination, 12, *107, 108, 248 
inversion (of chromosomes), *85 
investing bones, 230 
involution, *107, 108 ff, 112, 120 
iris, *266 

muscles, 239 
irradiation, 154, 173 
ischium, *235, 236 
isolecithal, 37, 90 
isthmus of brain, *311 

of yolk, *123 

Jacobson's organ, 263, 270 

jaws, 182 

Jenkinson, 157, 173 

Johannsen, 6 

jugal bone, *234, *235 

jugular ganglion, 258, *319 

vein, 223 


Kaestner, 162 
karyokinesis, 60, *61 
karyolymph, 32, 60 
karyoplasm, 32 
karyosome, see chromatin 
Kathariner, 156 
Keibel, 294, 317 

kidney, see pronephros, mesonephros, 

labia majora, 211 

minora, 211 
labio-scrotal folds, 211 
lagena, 267; see also cochlea 
lanugo, 247 

laryngeal cartilages, 233 

larynx, 191 

larva of amphioxus, 15 

of frog, 19, *130, 287 ff, *288 
latebra, *40 

lateral limiting sulci, 132, 147, *308, *314 
- line organs, 256, 263, 266 

mesoderm, *120 
laying (of hen's egg), 21 



lens (of eye), 130, *266, *284, *290, *309, 

pit, 264, *265, *305 

placode, 264, *265 

vesicle, 264, *265, *303, *312 
leucocyte, 216 

life history, see also ontogeny 

of amphioxus, 12, *14 

of chick, 19, *22 

of frog, 16, *17 

of man, 23, *24 

ligaments, 229 

ligation experiments, *155, *160, *161 

light, 173 

Lillie, 53, 165, 261, 294 

limb buds, 135 

of chick, *309, *314 

of frog (tadpole), 132 

of pig, *316, *322, *323, *326 

linkage, 78, *79 
liver, *182, 186, 222 

of chick, *190, *307, *313 

of frog, 188, *189, *276 ff 

of pig, H92, *316, *318, *323, *324, 

Loeb, 7, 56, 154 
lung, *182, 184, *190 

bud, *318, *323 
lymph space in frog, *291 
lymphatic system, 224, *225 


macrolecithal eggs, 37 

macromeres, 96, *103, 108 

male hormone, 206 

malleus, 234, *268, 269 

Malpighi, 4 

mammary gland, 246 

mammillary part (of hypothalamus), 252 

man, life history, 23, *24 

mandibular arch, 131, 182, 216, 233, 269 

Mangold, 162 

mantle layer (of spinal cord), 248, *253 

margin of overgrowth (of area opaca), 

112, *123, 248, *253 
Marshall, 210 

matrix (of chromosome), 61 
maturation divisions, 47, *50; see also 

maxilla, *234, *235 

maxillary ridge, 182 

meatus venosus, 226, *307, *313 

meckelian cartilage, 233 

mediastinum, 198 

medicine, 9 

medulla oblongata, see myelencephalon 

medullary, see neural 

sheath, 248 

megalecithal, see macrolecithal 
megaloblast, *205 

meiocyte, see 'cyte, oocyte, spermatocyte 
meiosis, 64, *65, *66 ff' see also matura- 
tion divisions 
melanin, 57, 246, 264 
membrane bones, 229 
membranous labyrinth (of ear), 267 
Mendel, 5, 73, 75, 78 
meninges, 250 
menstruation, 210 
meroblastic cleavage, *90, *172 
merogony, *155 
mesencephalon, 250, *251 

of chick, *299 ff, *309 ff 

of frog, *276, *277, *281, *283 

of pig, *319 
mesenchyme, 114, 229, *296 
mesendoderm, 107, 115 
mesenteric artery, *219 

vein, 222 

mesenteries, 196, *197 ff, *313 
mesentoderm, see mesendoderm 
mesoblast, see mesoderm, middle germ 


mesocardia, 196, *197, 218, *302, *307 
mesocoel, 252 
mesoderm, 89, *102, 106, 114 jf; see also 

middle germ layer 
mesodermal crescent, *55, 116 

derivatives, 116, 195 jf 

of chick, 199, 204, 212, 214, 226, 

237, 296, 300, 306, 313 
of frog, 198, 204, 212, 214, 224, 

237, 276, 282, 290 

of man, 199, 205, 212, 214, 227, 237 

of pig, 320 

mesoduodenum, 196 

mesogastrium, 196 

mesohepar, 196, *197 

mesonephric duct, 201, 208; see also 

Wolffian duct 



mesonephros, *200, *202, 233 

of chick, 204, *308, *311, *314 

of frog, 204 

of man, 205 

of pig, *316, *318, *323Jf, *314, *324 # 
mesophase, see metaphase 
mesorchium, 206 

mesovarium, 206 
metabolic nucleus, 60 
metacarpals, *235, 236 
rnetacoel, 252 
metamerism, 128 

of head, 260 

of nervous system, 258 
metamorphosis of Ambystoma, 175, *176 

of amphioxus, 15 

of frog, 19 

of spermatid, see transformation 
metanephric duct, 203, *326 
metanephros, *200, 202, *203 ff 

of pig, *192, *318 
metaphase, *61, 62, *66 
metaplasm, 34 
metatarsals, *235, 236 
metencephalon, *251, 252 

of chick, *304, *305, *309, *311 

of pig, *316, *318 
metestrum, 210 
methods of cleavage, 90 
micro-dissection, 165 
microlecithal eggs, 37 
micromeres, 96, *103 
micrometry, 353 
micron (/*), 353 
micropyle, *38 
microscope, 4, 350, *351 
mid-brain, see mesericephalon 
middle ear, 267, *268 

middle germ layer, 91, 106, 114 ff; see 
also chorda-mesoderm, mesoderm 

of amphioxus, *116, *117, *118 

of chick, *121 

of frog, 118, *120 

of man, 122 

mid-gut, 122, 127, 136, *181 

of chick, *297, *302, *308, *314 

of frog, *276Jf, *283jf 
milk ridge, 247, 317 
Miller's ovum, 25, *114, 122 
Milnes-Marshall, 42 

Minot, 317 

mitochondria, *32, 33, 48, 54, 55, *63 . 

mitosis, 60, *61, *65 

Moenkhaus, 156 

monoploid number (of chromosomes), see 


monovular twins, 163 
Morgan, 6, 81, 103, 156, 173 
morula, 23, 101, 106 
mosaic, 158, *159, 168 
motor-oculi nerve, see oculomotor 
mouth, 129, 182, *288 
mucous gland, see sucker 
Miiller, 88, 173 

Miillerian duct, 208; see also oviduct 
muscles, 238 ff, 269 

in tail of frog, *286, *289, *293 
mutation (of genes), 89 
myelencephalon, *251, 252 

of chick, *304, *309, *311, *312 

of frog, *291 

of pig, *316, *319 
myelocoel, 253 
myocardium, 218 
myocoel, 195 
myotome, *238 

of chick, *314 

of frog, *281 

of pig, *321 


nail, *245 
nares, 263 
nasal, see also olfactory 

bone, *234, *235 

cavity, 263 

pit, 130, 263 

of chick, *309, *312 

of frog., *288, *293 

of pig, *321, *327 

placode, 263 
nebenkern, see paranucleus 
neck, 129 

Necturus, 183, *240 

Needham, 7, 27, 37, 41, 137, 142 

neonychium, 247 

neoteny, 175, *176 

nephric ducts, 187, 200 ff 

- organs, 199 ff 

nephridia, 199 



nephrocoel, 195, 200 
nephrostome, 200, *201, 208 
nephrotome, 200 
nerve fibers, *254 
nerves, 253 ff 
nervous system, 247 ff 
neural crest, 248, 254, *302 

folds, 134, *297 

groove, 134, *295, *297 

plate, 120, 122, *248 

tube, 128, 248 

of chick, *296, *298, *303 

of frog, *276, *277, *279, *292 

neurenteric canal, *181, 188, *276, *277 

neuroblasts, 248 

neurocoel, 250 

neurocranium, 232 

neuromeres, 2(30 

neurons, 248, *249, *253, *254, *257, 259 

neuropore, *295, *299 

neurula, 18, 121, *168 

nictitating membrane, 270 

nodosum ganglion, 258, *320 

non-disjunction (of chromosomes), *85, 

*86, 87 

normal saline solution, 335 
normoblast, *205 
nose, 250, 261, *262, *263 
notochord, 89, 106, 114, 122 

of chick, *295, *297, *299 ff 

of frog, *276, *279, *283 ff 

of pig, *318-*327 
nuclear membrane, 60 

sap, see karyolymph 
nucleolus, *32, 60 
nucleoplasm, see karyoplasm 
nucleus, *32/ 

of brain, 252 

of Pander, see latebra 

pulposus, 231 
nurse cells, 52 


occipital bones, *234 
oculomotor nerve, *256, 266 
oesophagus, see esophagus 
oestrus, see estrus 
Okkelberg, 54 
olfactory, see also nasal 

capsule, 233 

olfactory lobe, 250, 263 

nerve, *256 

oligolecithal, see microlecithal 
omen turn, *197 

omphalomesenteric, see vitelline 
ontogeny, 3 

oocyte, 47, *51 
oogenesis, 43, 44, 47, 49 
oogonium, 46, *51, 207 
ootid, 48 

openings of body, 244 
opercular cavity, *291, *292 
operculum, 184, 188 
ophthalmic, see optic 
OppePs stain, 347 
optic capsule, 233 

chiasma, 256, *318 

cup, 264, *265, *281, *284, *303 ff, *320 

lobes, see corpora bigemina, corpora 

nerve, *256 

part of hypothalamus, 252 

placode, 248, 250, 264, *265 

recess (of pig), 326 

stalk, 264, *265, *280, *284 

vesicle, *128, 129, 264, *265, *276, 
*278, *299, *301 

oral cavity, 182 

gland, see sucker 

plate, 182 
organ fields, *168 

-forming substances, 163, *164 
organization, problems of, 154 ff, 156 ff, 

158 ff, 165 jf 
organizer/169, *170, 171 
organogenesis, see organogeny 
organogeny, 181 ff 
oro-nasal groove, *262, 263 
ossification, 229 
osteoblasts, 229 
ostium tubae, 208, *209 
otic, see also acoustic, auditory 

bones, *234, 267 

capsule, 233, 267 

pit, 130, 267, *268, *305 

placode, 267, *268, *302 

vesicle, 267, *268 

of chick, *303, *309, *312 

of frog, *276, *279, *281, *284, *291 

of pig, *319 



otocyst, see otic vesicle 

otoliths, 267 

outer ear, 267, *268 

ovary, *51, *206, 207 

oviduct, 208, *209; see also Mullerian 


oviparous, 51 
ovulatiori, 50, 51, 207 
ovum, see also egg, 43 

in ovo, 40 

period of, 134 

PI generation, 73 

palate, *263 

palatine bone, *234, 235 

pancreas, *182, 186 jf, *318 

parachordal bar, 232 

paradidymis, 208, *209 

paraffin, 339 

paranucleus, 48, *49 

parasphenoid bone, *234, 235 

parathyroids, *185, 188 ff 

paraurethral gland, 213 

parietal bones, *234, *235 

recess, 199 

paroophoron, 208, *209 

parthenogenesis, 151; see also artificial 

parturition, 151 
Patten, 121, 294, 317 
pecten, 270 
pectoral girdle, *235 
pelvic girdle, *235 
penetration (by sperm), 53 
penile urethra, 211 
penis, 211 
Perameles, 145 
perforatorium, see acrosome 
periblast, *105 
pericardial cavity, *197, *198, 218 

of chick, *300, *312 

of frog, *284 

of pig, *323 

perichondrium, 229 
periderm, *244 
perilymph, 267 
periosteum, 229 
peristomeal mesoderm, 115 
peritoneal cavity, *197, *198 

peritoneo-pericardial canal, 198 

peritoneum, 196 

petrosal ganglion, 258 

Pfliiger, 156 

phalanges, *235, 236 

phallus, 193, *211 

pharyngeal pouches, see visceral pouches 

pharynx, 183 

of chick, *304, *312 

of frog, *284, *290 

of pig, *316, *318 
phenotype, 75 
phrenic nerve, 239 
phylogeny, 6 

pia mater, 250, 264 

pig, 10 mm. embryo, 316-328 

pigment, 34, 55 

pineal gland, 250 

pinna (of ear), 269 

pituitary gland, 51, 233, 252; see also 

placenta, 142, 144, *149, *150 

of carnivores, *146, *149 

of marsupials, 144, *145 

of man, 25, 147, *149 

of ungulates, *145, *149 
placoid scale, 182, *230 
plasma membrane, *32, 33 
plasmasome, see nucleolus 
plasticity, 163, 165, 168 
plastid, *32, 34 

pleural cavity, *198 

groove, *314 
plica semilunaris, 271 
Plough, 89 

pneumogastric nerve, see vagus 
polar body, 13; see also polocyte 

furrow, 98 
polarity, 37, 156, 171 
pole, see polarity 
polocyte, 47, *50, *56, *57 
pons (of brain), 252 
postanal gut, *181 

postcardinal, see posterior cardinal vein 
postcaval vein, *223, 224, 228, *318, 

*323, *324, *325 
postcloacal gut, *187 
posterior cardinal vein, 200, 216, *217 

of chick, *307, *308, *313 

of pig, *322, *323, *325 



posterior caval vein, see postcaval vein 

chamber (of eye), *266 
precardinaly see anterior cardinal vein 
precaval vein, *223 

prechordal brain, 233, 250; see also 

precoracoid bone, *235, 236 
preformation, 4 
premaxilla, *234, *235 
Prentiss, 317 
preoral gut, *181, *318 
preotic myotomes, *239 
preserving embryos, 333 
pressure experiments, *162 
presumptive organ regions, *55, *56, 

*102, *104 
primitive groove, 121, *298 

node (or knot), *121, *297 

streak, 115, *121, *295, *296, *297, 

(of man), *124 

primordial germ cells, 45, *46, 205 

primordium, 12 

proamnion, 122, 138, *295, *297 

proctodeum, 127, 130, 132, *181, 187, 191 

proestrum, 210 

pronephrio duct, 200, *201 

pronephros, *200, *201 

of frog, *281, *285, *286, *288, *292, 

pronucleus, 52, 54, *56, 153, *155, 156 
prophase, 60, *61, *66 
prosencephalon, 250, *251 

of chick, *299, *300, *301, *303 

of frog, *276, *277, *280, *281, *283, 

prostate gland, 213 
prostatic utricle, *209, 213 
Protenor, *69 
protoplasm, 32 
Protopterus, *246 
pseudobranch, 184 
pseudopregnancy, 210 
pterygoid bones, *234 
pterygoquadrate cartilage, 233 
puberty, 26 
pubis, *235, 236 
pulmonary artery, *219, *220 

vein, 224 
Punnett square, 76, *77 


quadrate bone, *234, 269 
quadratojugal, *234, *235 


radialia (of carpus), 235 
radius, *235, 236 
rami of nerves, *253 
Rauber's cells, *113, 114, *141 
recapitulation theory, 6 
receptors, 247 
recessive genes, 73, 89 
'reconstruction (models), 357 
rectum, 187, *192 
reduction division, 67, 87 
regulation eggs, 158, *159 
renal, see also excretory, nephric 

artery, *221 

-portal vein, 226 
respiration, 183 
respiratory tree, 184 

resting nucleus, see metabolic nucleus 
retention theory, 8 
reticulum (of nucleus), 32, 60, *61 
retina, 264, *266, *290, *305 
rhinencephalon, see olfactory lobe 
rhombencephalon, 250, *251 

of chick, *299, *300, *302, *303 

of frog, *276, *277, *279, *281, *283, 

rhomboidal sinus, *299 
ribs, *231 

roots (of nerves), *253 
Roux, 6, 158, 161 
Rugh, 51, 332 
rules of cleavage, 89 


saccule (of ear), 267, *268 
Sachs, 90, 95 
sacral plexus, 250 
salivary glands, 183 
scales, *230, *245 
scapula, *235, 236 
Sceleporus, 230 
Schleiden, 5 
Schmidt, 159 
Schrader, 62 
Schwann, 5 



sclera (of eye), 264, *266 
sclerotome, 196, *238, *314 
scrotum, 211 
sebaceous gland, 246 
secondary embryo, *170 
secretory tubules, *202, 203 
segmental zone, 122 
segmentation cavity, see blastocoel 
segregation, 73, *74, *75 
self-differentiation, 168, *169 
semicircular canals, 267, *268, *291 
semilunar ganglion, 257, *319 
seminal vesicle, 212 
semination, 50, 51 
seminiferous tubules, *206 
sense organs, 132, 134, 233, 261 
septa of heart, 288 

septum transversum, *197, *198, *323 
serial sections, 339, *341 
sero-amniotic raphe, see amniotic raphe 
serosa, see chorion (of amniota) 
Sertoli cell, see sustentacular 
sex-chromosomes, *69, *70, *71 
sex-linked inheritance, 81, *83, *84 
Sharp, 32, 47 
shell gland, 212 

membrane, 38 
Siamese twins, 164 

sinus arteriosus (valves), *322 

of nose, *263 

rhombaidialis, see rhomboidal sinus 

terminalis, 216 

venosus, 218, *227, *303, *311, *327 
situs, inversus, 158 

skeletal labyrinth (of ear), 267 
skeletogenous regions, *229 
skeleton, 229 ff 
skin, see integument 

glands, *246 
skull, *232, *234, *235 
Smith's fluid, 334 
smooth muscles, 238, 254 
soma cells, 43, *45 

somatic layer of mesoderm, 120 

veins, 216 
somatopleure, 128, 136, 196 

in chick, *297 ff 
somites, 120, 122, *128 
- of chick, *295 ff 

of frog, *277# 

spawning of amphioxus, 13 

of frog, 13 
species hybrids, 156 
Spemann, 158, 161, 165, 169 
sperm, 34, *35, 42, 47 
spermatid, 47 

transformation, *49 
spermatocyte, 47 
spermatogenesis, 43, 44, 47, 48 
spermatogonia, 46, 206 
spermatozoon, see sperm 
spermioteleosis, see spermatid transfor- 

sphenoid bones, *234 

spheriolateral cartilage, *232, 233 

sphere substance, 48 

spina bifida, 162, 173 

spinal accessory nerve, see accessory nerve 

artery, *219, 221, *318 

canal, *283 

cord, 250, *253 

of chick, *307, *308, *311 ff 

of frog, *285, *293 

of pig, *320 ff 

ganglion, *253, *320 ff 

nerves, 250, *253, *254, *316, *321, *323 
spindle (mitotic), *61, 62 

spiracle, 131 

spiral coil (of sperm), *35 

splanchnic layer (of mesoderm), 121 

veins, 216 
splanchnocoel, see coelom 
splanchnocranium, *232, 233 
splanchnopleure, 128, 136, 142 

in chick, *297 ff 
spleen, 198, 215 
spongioblasts, 248 
Squalus, *230, *239 
squamosal bone, *234, *235 
stapedial muscle, 269 
stapes, 233, 267, *268, 269 
stem cell, *45 

sternum, 231, *232 

stomach, *182, *189, *192, *323, *327 

stomodeal plate, *304 

stomodeum, 127, 130, 132, 247, 252, *284 

Storer, 331 

stratum corneum, see corneum 

germinativum, see germinativum 

granulosum (of ovarian follicle), *207 



striated muscles, 238; see also muscles 

stroma (of testis), 206 

Sturtevant, 81 

Styela, 55, 93, *94, 159 

Subcardinal vein, *223, *323, *325, *327 

subcephalic pocket, 132, *296, *300, *301 

subclavian artery, *219, 220 

vein, 223 

subintestinal vein, 216, *225 

subnotochordal rod, see hypochord 

substituting bones, 230 

subunguis, *245 

sucker (of tadpole), 130, 247, *276^ 

sudoriparous glands, 246 

superfemale, 86, *87, 88 

superior ganglion, 258, *319 

supermale, *87, 88 

supersexes, 86, *87, 88 

supracardinal vein, 224 

suprarenal gland, *213, 214, 256 

sustentacular cells, 206 

Sutton, 5 

sweat glands, see sudoriparous glands 

Swingle, 214 

synapsis, 64, *65, *66, *81, *85 

syngamy, see fertilization 

tail, 129, 132, 135 

fin, *288 

fold, 132, 134, *303 

of amnion, 138 

tarsal bones, *235, *236 
taste buds, 183 

teeth, *182, 230 
telencephalon, 250, *251 

of chick, *304, *309, *311, *312 

of pig, *316, *318, *320 
telocoel, 251 
telolecithal eggs, 37, 92 
telophase, *61, 62, *66 
temperature gradient, 158, 162 
tensor tympani (muscle), 269 
teratology, 88 

terminal nerve, 256 
testis, *52, *206 
tetrad, 64, *65, *66 
thalamencephalon, 252 
thalamus, 251 

thigmotaxis, 53 

thymus, *185, *327 

thyroglossal duct, 184 

thyroid, 175, *182, *185, *192, *312 

thyroxin, 175 

tibia, *235, 236 

tongue, *182, 183, 239, *318, *320 

tonsil, 191 

torsion, 132, 157 

trabecula cartilage, *232 

trachea, *182, 183, *192, *316, *3l8jf 

translocation of chromosomes, *85 

transplantation, 165, *166 

trigeminal nerve, *256, 257 

triploid Drosophila, 88 

Triton, *103, 153, 154, *155, 162, H63, 

*170, 174, *175 
trochlear nerve, *256, 266 
trophoblast, 42, *106, *113, 144* 
trophoderm, see trophoblast 
tuberculum impar, 191 

posterius, 278 

tubo-tympanic cavity, 187, 189, 191 
tunica albuginea, 206, 208 
tympanic cavity, 184, 269 
tympanum, *268, 269, *270 


ulna, *235, 236 
ultimobranchial bodies, *185 
umbilical artery, 135, *219, 221, *318, 
*325, *326 

blood vessels, 144 

cord, *141, 147 

stalk, 132, 137, *139, 140, *148 

vein, 135, 222, *324, *325 
unguis, *245 

ureter, 203 

urethra, 193, 213 

urinary bladder, 187, *189, 204 

urodeum (of chick), 191 

urogenital aperture, 187, 193 

ridge, 212 

sinus, 187, 193 

system, see genital organs, nephric 

uterine milk, 135, 210 

uterus, 212 

utricle (of ear), see vestibule 

utriculus prosiaticus, see prostatic utricle 



vacuoles, *32, 34 

vagina, 212 

vagus nerve, *256, 258, 267 

van deferens, see deferent duct 

vascular system, 214 ff 

of chick, 226, *227 

of frog, 226, *227 

of man, 227, *228 

vegetal pole, see pole 

veins, 221 ff 

velum transversum, 326 

vena caw, see precaval vein, postcaval 


vent, 187 

ventral aorta, 216, *217, *219, *220 
ventricle, 218 

of chick, *303, *307, *309, *311, *313 

of frog, *284 

of pig, *318, *322, *323 
vermiform appendix, 192 
vertebrae, 230, *231 
vertebral artery, *219, 221 

vein, 223 

vesico-urethral sinus, 193 
vestibular nerve, 267 
vestibule, 267, *268 
villi, 146, *149 

visceral arches, 132, *240, *257 

of chick, *305, *312 

- of frog, *286 

of pig, *320 

clefts, 6, *128, 130, 134, 233 
of chick, *303, *309 

grooves, 130 ff, 269 
of chick, *305 

muscles, *240, *241 

pouches, 130, 131, 134, *182 
of chick, *305 

of frog, *276, *286 

of pig, *320, *327 

Itamins, 176 
vitelline artery, 135, *217 

of chick, *303, *309 

of pig, *318, *324, *325 

membrane, 34, 38, 53, *57 

vitelline vein, 135, 185, 199, 216, *217, 

of chick, *297, *299, *302, *303, 

*307, *309 

of pig, *318, *325 

vitreous humor (or body), 264, *266 
viviparous, 51 
Vogt, 102, 103, 118 
vomer, *234, 235 
von Baer, 4 


W-chromosome, 71 

water, 173 

Weismann, 7, 43 

Whitman, 5 

Wilson, 159 

Witschi, 161 

whole mounts, 336 

Wolff, 4 

Wolffian body, see mesonephros 

duct, 208 ff; see also mesonephric 

Wright, 331 

X-chrotnosome, 69 ff, *70, *71, *72, 81 ff 
xenoplastic transplantation, 165, *166, 
*167, 171 

Y-chromosome, 70 ff, *71, *72, 81 jf 
yellow crescent (of Styela), 55 
yolk, 34, 37, 55, 92, *126 

in frog's egg, 135, *277, *281, *283, 

nucleus complex, 50 

plug, *110, 111 

sac, 114, 135 ff, 144, *305 
umbilicus, 136, *139, 143 

Z-chromosome, 71, *72 

zone of junction, *123 

zygote, 31; see also fertilized egg